WARNING for chemicals

"During the last ten years while the material presented in this manual was being edited, illustrated, laid out, translated and finally printed, there has been a growing awareness of the potential danger of some of the chemicals commonly used for semi-intensive and intensive farmed fish management. In particular, malachite green mentioned in Chapter 15 has been legally banned in certain countries because of its very long-lasting residual presence in the treated fish." (Towards safe and effective use of chemicals in coastal aquaculture. GESAMP Reports and Studies No. 65. Rome, FAO, 1997.)

15.0   Introduction

1. Fish diseases may cause severe losses on fish farms through:

2. While it may be difficult to avoid fish diseases completely, it is better to try to prevent their occurrence rather than to allow them to develop and then attempting to cure them once they start to cause problems (see Section 15.2). To cure a fish disease is much more difficult and generally calls for the services of a specialist. By the time proper treatment can be organized, the disease may have become more serious. In some cases surviving fish are so weakened that effective treatment becomes difficult.

3. However several simple and effective treatments can be used, either for prevention or early control of disease before it becomes too serious. These methods are described in Section 15.3. In all cases, you will have to handle chemicals and know about their dangers and their use (see Section 15.1).

Main causes of disease in farm fish

4. There are several causes of disease that may affect the fish directly or may continue to cause disease problems. Basically, any factor which causes stress or difficulty to the fish decreases its resistance to disease and increases the chance of disease problems occurring.

5. The three main causes of disease are:

(a) Fish are not fed properly: nutritional diseases become more frequent as the culture system becomes more intensive and the fish obtain less of their nutrients from natural food organisms (see Chapter 10).

(b) Fish are stressed by being exposed to an extreme or a toxic condition: in the previous chapters, you have already learned about such factors as:  

(c) Fish are attacked by successful disease organisms, either externally on the skin, gills or fins, or internally in the blood, digestive tract, nervous system, etc. You will learn more about these organisms in Section 15.3.

6. As the characteristics and management of the first two causes of disease have already been fully discussed earlier, the next sections will concentrate on the prevention and simple treatment of fish diseases caused directly by living organisms.

Life cycle of lchthyophthirius multifilis responsible for the white-spot disease

This disease may spread rapidly from one fish to another through water and pond bottom infections which makes disease control very difficult

How does disease caused by living organisms develop in fish ponds

7. The development of disease caused by living organisms is encouraged by any condition which places the fish under stress.

8. Disease risks become even greater when fish undergo combined stresses, for example handling when the water temperature is below normal or overcrowding in low dissolved oxygen conditions.

9. Other factors on the fish farm may also be responsible for the survival and propagation of disease organisms, making disease control much more difficult such as:

Life cycle of Diplostomum spathaceum

Preventing diseases through good management

10. As already explained, disease prevention on the fish farm is better than cure. All efforts should thus be directed to applying good management practices.

11. Be particularly vigilant about the following points.

(a) Ensure good water quality: sufficient supply, with adequate dissolved oxygen concentration and free of pollution (see Chapter 2).

(b) Keep the pond environment healthy: control silt (see Section 11.6, Construction, 20),control plants (see Section 4.9),keep a healthy balance of phytoplankton and zooplankton (see Section 10.1), and exchange water if needed. If necessary, use mechanical aeration (see Section 2.8).Disinfect the pond regularly (see Section 15.2).

(c) Keep the fish in good condition:control stocking density. Keep different sizes or sexes separate if necessary to control fighting. Ensure good food supply (see Chapter 10). Handle the fish properly, especially during harvesting (see Chapter 11) and sorting/grading (see Chapter 12). Care for your fish during storage (see Chapter 13) and transport (see Chapter 14).

(d) Prevent the entry of disease organisms from outside your farm:

(e) Prevent the spread of disease organisms within your farm: 

15.1   Common chemicals and their use on the fish farm

1. Most chemicals used for controlling disease organisms are toxic and/or irritant for the skin and respiratory tract. Many chemicals can cause serious health problems if swallowed or absorbed through the skin. You should therefore handle chemicals with care, mark them clearly and store them safely, away from children in particular.

2. When handling chemicals take at least the following precautions:

3. Further precautions for handling pond disinfectants in the open are given in Section 4.6.

Buying and storing chemicals

4. Most chemicals degrade with time. When buying chemicals ensure that they are still of good quality. Check their expiration or "sell by" date, if any. Check also whether they have been stored properly. Clearly indicate on each container the date of purchase.

5. For storing chemicals, use a cool, dark, dry and lockable room. Some chemicals may require refrigerator storage; check labels or handling instructions. Generally, chemicals deteriorate more quickly in warm conditions. Keep good records and ensure a good rotation of the stocks.

Determining the strength of chemicals and their solutions

6. Treatment chemicals are costly and can, in the wrong dosage, be toxic to fish. It is therefore essential to know and understand how the strength of these chemicals is expressed and how dosages can be calculated. You should become familiar with concentrations of chemical solutions, treatment units and their conversion values. You will then be able to avoid wasting chemicals and losing fish.

7. Treatment chemicals contain one or more toxic ingredients to disable or kill disease organisms. These are called the active ingredients (AI). The amount of active ingredient contained depends on the chemical. The AI concentration is expressed in percent of the total weight or volume of the chemical.


8. The usual practice is to dilute solid or liquid chemicals in water and prepare:

9. These solutions are prepared to a recommended strength or concentration, expressed either as:

10. Be very careful to make sure whether it is the chemical or the active ingredient which is being referred to. Examples are given later for making up these solutions. The concentration of material (chemical or active ingredient) in a solution may be expressed in various ways:

(a) As the weight present in the solution volume, such as milligrams per litre solution (mg/l), grams per cubic metre (1000 l) solution (g/m3), grams per 25 l solution (g/25 l);

(b) As a volume present in the solution volume such as millilitres per litre solution (ml/l), millilitres per cubic metre solution (mI/m3), millilitres per 10 l solution (ml/10 l).

In practice, you may consider that:

1 ml water weighs 1 g
1 l    water weighs 1 kg

(c) As a percentage, the number of parts (normally by weight) of material in 100 parts of solution.


 A 1.5 percent solution of salt contains 1.5 g salt per 100 g = 100 ml solution or 15 g/I; a 5 percent solution of chlorine bleach powder contains 5 g chlorine per 100 g = 100 ml solution or 50 g/I.

(d) In parts per million (ppm), the parts of material present in one million parts of solution; by weight this is equivalent to mg/l, g/m3, or by volume to ml/m3 of solution.


A 1000 ppm solution of chlorine bleach powder is in fact a 1000 mg/l solution; this amounts to 1 g chlorine per litre; a 160 ppm solution of formalin contains 160 mg/m3 of formalin; this is equivalent to 0.16 ml formalin per litre solution.

(e) As a ratio, the parts of solution per each part of Al, such as a 1: 4 000 solution where for example each ml Al is present in 4 000 ml solution (1 ml/4 l).

Note: to obtain a ppm value from a ratio, multiply the ratio by 1 000 000 as shown in the chart below.

Calculating the amount of chemical to be used

11. As explained earlier, you may wish to make a stock solution, a working solution or a treatment dosage of the chemical concerned. The concentration required will be determined by the type of chemical and treatment used (see Sections 15.2 and 15.3), and the amounts used will depend on the concentration and the overall volume of the container, tank or pond involved.

12. Calculations of the amount to be used depend on the way the dosage is expressed (see paragraphs 9 and 10) and on the percentage of Al present in the chemicals.

13. If you are using a chemical with 100 percent Al, proceed in one of the following ways.

(a) When the concentration is given as a weight or volume of Al per volume of solution, multiply this concentration by the total volume of water to be treated.


(b) When the concentration is given as a percentage, multiply this concentration (expressed as a decimal number) by 1000 times the water volume (in l) to obtain the amount of chemical in ml or g.


Recommended treatment dosage is 2 percent common salt. Your plastic barrel contains 30 l water; you will need 0.02 x 1000 x 30 l = 600 g salt.

(c) When the concentration is given in parts per million (ppm), multiply this concentration by the water volume (in l). Divide the result by 1000 to obtain the amount of chemical in ml or g.


Recommended dosage is 100 ppm copper sulphate. Your trough contains 500 l water. You will need (100 ppm x 500 l) 1000 = 50 g copper sulphate.

(d) When the concentration is given as a ratio, multiply this concentration by 1000 times the water volume (in l) to obtain the amount of chemical in ml or g.


Recommended concentration is 1: 6000 formalin; your tank contains 2000 l water; you will need (1 6000) x 1000 x 2000 l = about 333 ml of formalin.

14. If you are using a chemical that contains less than 100 percent active ingredient, divide the result obtained from the above calculations by the percentage of Al (expressed as a decimal) in the chemical.


Recommended dosage of an insecticide containing 80 percent Al is 0.5 ppm Al. Your pond contains 50 m3 = 50000 l water:

15. In the next sections, you will learn how to apply the above calculations on your fish farm. To do this, you need to know:

Measuring chemicals

16. To weigh chemical powder, crystals, etc., you require a balance with good accuracy within a particular range.

17. For small quantities, you may use, for example, a simple microbeam balance or a small spring balance, with good accuracy within the range of 1 to 125 g.

18. For larger quantities, you will need a good beam or spring balance similar to the one used to weigh fish (see Section 8.6), accurate within the range of 100 g to a few kilograms, depending on the volumes of water you plan to treat.

19. For quantities smaller than 1 g, use stock or working solutions (see paragraphs 25 to 27).

20. To measure volumes of liquid chemicals, you require a series of graduated plastic cylinders, with a typical range of capacity from 10 ml to 500 ml. The smaller the individual capacity of the cylinder, the more accurate the measurement.

Note: you can get this equipment, either from school, medical or laboratory suppliers or from photographic supply stores.

21. For small volume measurements of one or a few millilitres, it is best to use either a medical syringe (capacity 2 to 5 ml) or a graduated or bulb-type pipette. Obtain these from a local pharmacy, drugstore or medical office.

22. For volumes smaller than 1 ml, use stock or working solutions (see paragraphs 25 to 27).

Determining water volumes

23. In a previous manual in this series (see Section 2.0, Water, 4), you learned how to determine the volume of water in a pond by multiplying its surface area by the average water depth. Keep in mind that both these values may change as the pond ages. Be sure to adjust your estimate of water volume accordingly before treating any pond.

24. To determine the volume of other structures or of various containers, proceed as follows.

(a) For circular tanks or drums, multiply area of base (in m2 ) by water depth (in m). Calculate area of base as 3.14 x (D2 4) where D (in m) is the inside diameter of the base.

(b) For a rectangular trough/tank or container, multiply base area (in m2 ) by water depth (in m). Calculate base area as inside length (in m) times inside width (in m) of base.

(c) For rectangular tanks with sloping sides, use average width and length (at half water depth) to calculate area.

(d) For fully filled barrel-shaped containers, calculate the inside midsection area and the base area. Get the average value of the two (in m2) . Multiply this by the water depth (in m).

(e) For circular tanks or drums with sloping sides, use the average area as for barrel-shaped containers, using the diameter at the base and at the upper water level. Multiply the average area by the water depth.

(f) For partially filled barrels:

(g) For small containers or those with an irregular shape: determine water volume by filling them up to the desired water depth with a container of known volume such as a bottle, a tin or a bucket.

Note: it may he helpful to mark, using paint or indelible pen, the water levels corresponding to the water volumes you normally use. Good records will allow you to dose quickly and accurately for specific water volumes.  


Using stock and working solutions

25. The commonly available balances and measuring cylinders are not accurate enough to measure very small amounts of chemicals such as fractions of a gram or millilitre. It is best then to prepare a stronger stock solution, from which the final solution is prepared. The stock solution can usually be stored, to use as required. In some cases, you can dilute the stock solution to a working solution, which can be used for a short time, for example during one or two days, to be diluted as needed.

26. Stock solutions and working solutions are usually made up to concentrations which make them convenient to use, for example:

Preparing stock and working solutions

27. You can either:


You are treating fish stock in 50-l buckets, and you need a treatment dosage of 1 mg/l. You have a 20-ml beaker for dosing the chemical. What strength of solution do you need to give the correct dosage, using one beaker full for every bucket?

The 50 l bucket requires 50 l x 1 mg/l = 50 mg of treatment chemical. Thus you need 50 mg in each 20 ml beaker. Concentration of required solution is therefore 50 mg/20 ml = 50 mg x (1000 ml 20 ml) = 2 500 mg/l or 2.5 g/I.

Note: do not try to make concentrations stronger than the solubility limits of the chemical; the supplier will have this information. With dissolving solids, you can visibly check the solubility limit as excess solids will remain suspended in the liquid. In general, however, you should try to make the stock solution quite concentrated, as it will usually keep better and it will be less affected by accidental contamination.

Storing and dispensing stock and working solutions

28. Stock and working solutions should be stored in clean bottles, preferably made of dark glass. You may also darken a clearer glass or plastic bottle with black tape. Label the bottle clearly, indicating the kind of chemical, its date of preparation, its strength and notes for its use, for example: malachite green stock solution, prepared 6/6/90, 100 g/I - use 2 ml/100 I to make 2 mg/l solution.

29. Some chemicals react with other materials, particularly cheap household plastics. Do not use these materials if possible.

30. Bottles should be well stopped preferably with a glass, rubber or cork top that will not react with the ingredients.

31. Store the chemicals in a cool place, refrigerated if necessary.

32. When measuring out the stock or working solution, pour a sample in a clean glass beaker to make sure it has not deteriorated. If acceptable, pour out approximately the amount required, then measure it exactly, using a clean pipette, syringe or measuring cylinder.

33. Alternatively, draw out the required amount with a clean pipette or syringe, checking to ensure its quality is still acceptable.

34. As unused working solution might have become contaminated by the measuring equipment or by exposure to the open air, it is preferable not to return it to the stock solution.

Common chemicals to prevent and cure fish diseases
(P used as preventive, see Section 15.2; C used as curative, see Section 15.3)

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Some common chemicals for fish farmers

35. There are several chemicals which are commonly used by fish farmers to prevent and cure fish diseases (as shown in Table 39).

36. Limes and calcium cyanamide are particularly useful to control pests in drained ponds (see Section 4.6).

37. Agro-industrial by-products such as rice bran, molasses and tobacco dust shavings can be used for similar purposes (see Section 4.6).

38. Organic poisons such as rotenone and saponin can control pests in undrained ponds (see Section 4.7).

39. Household bleach consists of a weak solution of sodium hypochlorite, which can be used as a good, general disinfectant of non-metallic equipment and working areas. As the active chlorine readily evaporates, a fresh solution should be used within two days.

40. Chlorine bleach liquid is a stronger solution containing 13 percent active chlorine. It can be diluted or used directly as a strong disinfectant, for example for sterilizing tanks.

41. Chlorine bleach powder contains 33 percent active chlorine. It makes a very strong disinfectant, and is especially useful for tanks.

42. lodophores are organic iodine compounds sold as a brownish liquid under various trade names such as Wescodyne, Romeiod, FAM 30 and Buffodine. They are excellent disinfectants, but highly toxic to fish. The active ingredient content varies from one make to another, so check carefully before use. A concentrated solution can be stored for several months in a cool, dark place. The diluted solution remains active for up to a week, until it fades from a brown to straw colour.

43. Benzalkonlum chlorides are a blend of quaternary ammonium compounds generally sold either as a powder or as a solution under various trade names such as Roccal (10 to 50 percent Al) and Hyamine 3500 (50 percent Al). These are very good disinfectants which can be reused for up to one week.

44. Common salt as used in the kitchen is usually a cheap and easily available chemical (sodium chloride). In solution, it not only kills several disease organisms but may also have positive effects on the fish by stimulating appetite and increasing mucus secretion, improving resistance to handling. Depending on the species, excess levels may however stress the fish. Thus cyprinids are more susceptible than salmonids.

45. Formalin is the commercial name for a 35 to 40 percent solution of formaldehyde gas in water. Avoid solutions containing methanol. Formalin is toxic to fish particularly in soft water (Section 5). As it lowers dissolved oxygen levels, make sure treatment water is well oxygenated. Toxic and irritant for the eyes and lungs, it should be handled very carefully in a well-aerated place. It is very sensitive to light so should be stored in a dark bottle. Always check to see if there is a white deposit at the bottom of the bottle. In such cases, before use, carefully filter out this highly toxic deposit of paraformaldehyde. Keep formalin away from metallic equipment. Remember when determining how much to use that it is considered to be a 100 percent Al chemical.

46. Malachite green is sold either as a blue-green to green crystalline powder or as a solution of various strengths. You should use the medical-grade, zinc- free quality to avoid heavy fish losses. Do not use in the presence of zinc or iron. Handle carefully and avoid any contact with your skin. If possible, use a trial batch, as quality and toxicity may vary greatly from one batch to another. Not to be used on food fish.

Note: in warm climates, it is safer to use a low dosage mixture of formalin and malachite green, called the Lateux-Meyer mixture. These two chemicals may be mixed and stored together before use.

47. Potassium permanganate is a violet, crystallized powder. It is a good disinfectant in the absence of organic materials that destroy it. When in solution, it should be kept in a dark bottle.

48. Copper sulphate is sold as a light-blue powder which readily dissolves in water; blue crystals are also common, but they should be small enough to be easily soluble; it is relatively cheap, but is highly toxic for humans and fish. It should be stored safely and handled properly (see also Section 4.9).

49. Agricultural insecticides are usually organophosphates and are sold under various trade names such as Bromex, Dipterex, Dylox, Flibol, Masoten and Neguvon. Carefully check the percentage of active ingredients present. They are often very toxic for various other aquatic organisms including zooplankton and for humans and domestic animals. In ponds, they normally decompose within a few days. Toxicity to disease organisms is often reduced when water temperature exceeds 30 C.  

How does the toxicity of these chemicals vary

50. Always remember that the toxicity of these chemicals for fish may vary greatly according to water quality. In general toxicity increases:

51. Check your water quality and look at the chart that follows for the chemical you plan to use. You will learn more about toxicity when considering the choice of disease treatment (see Section 15.3).

Variation of toxicity according to water quality
(l toxicity Increases; D toxicity decreases; 0 no effect)

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Note: as the water quality factor increases, the toxicity varies as indicated.

Using these chemicals to treat fish

52. Use of the first group of chemicals in Table 39 is described earlier (see Sections 4.6 and 4.7). For the others there are three common approaches.

53. In the dip treatment, fish are kept in a relatively strong solution of the chemical for a very short time, usually less than one minute. Proceed as follows:

54. In the bath treatment, fish are kept in a weaker standing solution of the chemical for a longer period of time, which may last from a few minutes to one hour (short bath) in a medium concentration, and up to 24 to 48 h (long bath) in a very low concentration. Proceed as follows.

(a) For a short bath, mix with water in a plastic watering can the amount of chemical required for the water volume to be treated. Lower the water level in the trough or circular tank by one-third to one-half. Let the water flow in again while spreading the previously mixed chemical over the entire surface area.  If necessary, mix the water with a clean broom, an agitator or an aerator to evenly disperse the chemical within the whole water mass. Stop the water inflow once the water reaches its normal level. Treat for the required duration. Then drain two-thirds of the water while starting the water flow again.

(b) For a long bath, stop the water flowing into the tank or pond. Drain some water and reduce water volume to the acceptable minimum for the stock density and water temperature conditions. Determine water volume and required amount of chemical. Dilute this chemical amount at least 100 times, for example in several plastic buckets before application. Add this diluted solution to the tank or pond, spreading it as much as possible over the entire surface area and mixing it well with the pond water. Treat for the required duration. Then open the water flow and raise the water level back to normal. It necessary, drain again and refill.

(c) If you are using a chemical which strongly colours the water such as malachite green, you may open the inlet, pour the already diluted chemical in the inflowing water and follow it as it moves across and through the tank or pond. When the colour appears at the outlet, close both inflow and outlet. When the treatment is finished, proceed as before.

55. In the swab treatment, the concentrated chemical is applied directly to the individual fish, usually with cotton wool or a sponge. This method is usually only used for valuable broodstock fish with specific sores or wounds. The fish should be handled very carefully to avoid the ill effects of additional stress.

56. There are two other ways to use chemicals but they are more difficult to apply properly and control safely:

15.2   Prevention of diseases through disinfection

1- As you have already learned, prevention of disease is always better than cure; good disinfection is the most important factor in preventing the entry and spread of disease in the farm. In this section, you will learn how to disinfect equipment, tanks and ponds, fish eggs, juvenile fish and broodstock.

Disinfecting equipment on fish farms

2. For safe and hygienic practice, the following equipment should be regularly disinfected:

3. To disinfect this equipment properly proceed as follows.

(a) Clean it well by brushing and rinsing.

(b) Apply one of the chemical solutions shown in the following chart, either with a sponge or a brush; whenever possible, immerse the equipment fully into the solution. Preferably use protective gloves.

(c) Wait for 10 to 15 minutes.

(d) Rinse thoroughly several times to remove toxicity, before using for fish.

Disinfection of equipment
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Disinfecting tanks

4. Rearing, storage and transport tanks can be disinfected with chemicals as shown in the chart below. Clean the tank thoroughly, especially in corners and drain areas. Apply the recommended solution either by hand with a brush or sponge or using an agricultural sprayer. Protect yourself properly, with gloves, goggles, mask and waders, especially while spraying.

Disinfection of tanks
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Disinfecting earthen ponds

5. You have already learned earlier:

Disinfecting fish eggs

6. Fish eggs can be major sources of transfer of disease from infected broodstock to fry and fingerlings. For this reason, all fish eggs should be thoroughly disinfected before transfer to other facilities. For best results, proceed as follows:

(a) Freshly prepare a 50 ppm Al solution of an iodophore chemical, for example:

You will need plenty of this solution, at least 40 l for 100 000 eggs.  

(b) Adjust the pH of this solution to about 7 with a suitable buffer solution such as sodium bicarbonate (about 100 mg/l).

(c) Adjust the temperature of this solution to the incubation temperature of the eggs (see Section 9.3).

(d) Use this solution to give a short bath of 10 minutes duration to each batch of eggs. When the solution turns yellow, replace it with a fresh one.

(e) Rinse the eggs well three times with clean water at the incubation temperature.

(f) Return the eggs to continue their incubation.

Note: fish eggs can be treated this way either immediately after fertilization or, more frequently, when they reach the eyed stage (see Section 9.3).

7. If you run your own hatchery, it is best to treat your eggs regularly against the very common infection caused by the fungus Saprolegnia (see Section 15.3). There are several treatments, depending on the type of incubator you use.

(a) if you incubate eggs in a vertical jar (see Section 9.3) proceed as follows:

(b) If you incubate eggs in a trough, proceed in a similar way. Stop the water inflow and treat the eggs with a 1 ppm malachite green solution for about five minutes.

(c) If you cannot treat the eggs within the incubator itself, you should wait until they have developed to the eyed stage. You may then carefully remove the eggs to give them a short bath in a malachite green solution:

BEWARE: it is always safer to carry out preliminary tests when using a batch of chemicals or a fish species for the first time to determine when and how disinfection can safely be carried out (see Section 15.3).

Disinfecting broodstock

8. Whenever you import new broodstock into your fish farm, it should be disinfected on arrival. Proceed as follows.

(a) Store the new broodstock in a tank or a small pond, kept separate from the other areas, and with a water supply which does not drain into other rearing units.

(b) Give a bath of potassium permanganate for one hour, at the dosage of 5 to 10 ppm according to water quality.

(c) Two days later, give a bath of formalin for at least four hours at the dosage of 10 to 15 ppm according to water quality. Alternatively, you may also use a mixed solution of malachite green and formalin (3.3 g malachite green/I formalin) for one hour at the dosage of 25 ppm.

9. A simpler but usually less effective way is to use a short bath of 10 minutes in a 2.5 percent salt solution.

10. After breeding, fish usually become rather susceptible to the development of fungi such as Saprolegnia (see Section 15.3). You can prevent this from happening through disinfection of the water where you keep your spent broodstock:

Note: for all treatments of fish with chemicals, follow the standard precautions both before and during treatment, as described in Section 15.3.

Disinfecting juvenile fish before stocking

11. When juvenile fish are produced in more intensive systems such as modern hatcheries, it is best to systematically disinfect them before stocking. According to the culture system, this is routinely done for both fry and fingerlings:

12. To disinfect juvenile fish during storage, proceed as follows.

(a) Prepare the chemical solution in a wide-mouthed plastic container:

Note: prepare a stock solution of malachite green by dissolving 2 g per 10 l of water. Then use 0.5 ml of this stock solution per litre of water to obtain a 0.1 ppm solution.

(b) Place a pocket of fine-mesh netting within the container.

(c) Treat successive batches of fish in this netting:

(d) Then return the fish to clean and well-aerated water for storage until their transport (see Section 13.2).

13. To disinfect juvenile fish during transport prepare in advance, for example in a plastic bag or bottle, the. amount of chemical(s) required to treat the volume of water present in the transport container.

14. If the fish are directly stocked from the container into a pond without further handling, stop the transport three to four minutes before reaching the pond. Pour the chemical solution into the container and dissolve it well into the water. Start the transport again and stock the fish into the pond immediately upon arrival.

15. If the fish need to be handled for stocking, pour the chemical(s) into the container three to four minutes before starting to stock the fish into the pond.

Note: follow the standard precautions both before and during treatment, as described below in Section 15.3.

15.3   Fish diseases and possible cures

Disease symptoms in fish

1. Apart from obvious signs such as dead or dying fish, there are many other symptoms which show that fish are not healthy:

2. Observe your fish frequently, for example while feeding them. Learn how they behave and look underwater when healthy to be able to detect as early as possible that something is going wrong. Use the following chart to help you.

Common disease symptoms in fish
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Identifying the cause of disease

3. It is not easy to identify in a fish pond why fish show signs of bad health. There are two common situations which you should readily recognize.

(a) A large part (if not all) of the fish stock show distress or die suddenly, with only some of the above symptoms of disease such as gasping at the surface or gaping mouths: the cause is prior stress (for example rough or poor handling or transport) and/or bad water quality(often low dissolved oxygen) or the presence of a toxic material such as pesticides or other pollution. You have learned earlier what the signs and possible causes are for low oxygen (see Section 2.5).

(b) Only a few fish are dead while some others show distress. Usually a few fish die over a period of several weeks and some of the above symptoms are present. The cause is improper feeding and/or development of some disease organism. Examine the fish closely, observing their swimming behaviour and response to feeding. Net a few suspect fish out of the pond and look for visible physical signs of disease, using if possible a magnifying glass. Take samples from skin and gills to be examined under a microscope, either at the farm (see paragraph 14) or in an easily accessible veterinary/medical laboratory.

4. If in doubt, do not hesitate to call on specialized assistance, for example from a veterinary extension service. Make a careful record of all the circumstances, to provide as much information as possible to assist specialist diagnosis.  

Which living organisms cause diseases in fish

5. There are three major groups of living organisms that may be responsible for fish diseases:

6. Viruses are practically invisible organisms. Their detection and identification requires highly specialized laboratory techniques. Control of viral diseases is difficult and requires specialized advice.

7. Bacteria are minute single-cell organisms (I to 12 m), usually living in colonies. Their detection and identification generally also require special laboratory techniques. The treatment of bacterial diseases such as tail or fin rot and skin ulcers requires experienced, specialized advice.

Bacterial diseases

8. Parasites are very small to small organisms made up of one or several cells. They develop either inside or outside the body.

(a) Internal fish parasites are very difficult to control. Although their effects can sometimes be easily identified, detection and identification of the parasites themselves usually requires special skills. Some examples are Myxosoma cerebralis, protozoan cysts in the head of trout causing whirling disease; larval stages (metacercariae) of sucking worms (trematodes), causing black-spot disease and blindness, and tapeworms (Ligula sp.) in the body cavity.

(b) External fish parasites are much easier to detect and identify. It is usually possible to eliminate them with an appropriate chemical treatment. In the next sections you will learn more about them.

The most common external fish parasites

9. The most commonly encountered external parasites on a fish farm belong to six different groups.

(a) Protozoa are very small, single-cell parasites such as:

 Note: 1 m = 0.001 mm = one thousandth of one millimetre.

(b) Flukes (Monogenea) are very small worms attached by hooks, such as Gyrodactylus (body fluke) and Dactylogyrus (gill fluke); size 0.3 to 1 mm.

(c) Leeches are rather large, segmented worms attached by a sucker on each end, such as Piscicola sp. (3 to 5 cm).

(d) Copepods often have two elongated egg sacs attached such as Lernaea sp. (anchor worm, 5 to 20 mm) and Ergasilus sp. (size 0.7 to 1.7 mm). Larval stages are similar to the non-parasitic copepods of the zooplankton (see Section 10.1).

(e) Fish lice (Crustacea) have a flat, disc-like body covered by a rounded dorsal carapace such as Argulus sp. (6 to 10 mm).

(f) Water fungi (water moulds) are made of filaments that usually grow into a cotton-like mass or mat such as with Saprolegnia sp.; they can also develop in the gills (Brachiomyces sp.).

Common external parasites of farmed fish





Locating these external parasites on the fish

10. Look for these external parasites on the body skin, the gills, the fins and in the mouth. The base of the fins is a favoured place of attachment.

11. Some of these parasites are mostly (if not exclusively) found on a particular organ of the fish as shown in the following chart. Some of them also develop on fish eggs, as described earlier (see Section 15.2).

Location of most common external parasites on fish1

Looking for and identifying external fish parasites

12. Depending on their size (see the chart above), you will require:

13. You can obtain relatively cheap pocket magnifiers from suppliers selling scientific or optical equipment. More powerful lenses are also available but are more expensive and difficult to use on the farm.

14. If you have quite a large fish farm, or if you can group together with neighbouring farmers, it would be worthwhile to invest in a good microscope and to learn how to use it properly. You could get assistance in selecting and using a microscope from a local medical or veterinary service, from a farm extension service, or from a local secondary school or college teacher. When buying a microscope, ensure that it has the following items:

15. In addition you should obtain some glass microscope slides (about 2.5 x 7.5 cm and 1 mm thick) and cover-slips (clear glass, about 2 x 2 cm and 0.2 mm thick).

A good microscope for a fish farm

A Binocular head with two eyepieces (10x) 
B Three objective lenses (10x,25x,40x)
C Standard stage (mechanical if possible)
D Condenser
E Mirror to reflect light (natural or artificial)

16. To look for and identify external parasites with a microscope, proceed as follows.

(a) For skin/scale samples, collect the mucus (slime) material from the surface of the fish by scraping against the run of the scales.

(b) Take samples from several skin areas, particularly where the skin is reddened, or where the mucus has a greyish, opaque appearance.

(c) Place each separate scraping in the centre of a clean glass slide. Add a drop of water and place a cover-slip on top.

(d) For gill samples, remove a gill arch and cut off the red parts (lamellae). Place some in the centre of a clean glass slide. Add a drop of water and place a cover-slip on top.

(e) Examine these fresh samples immediately under the microscope under low magnification (objective 10x). Look for signs of movement, which are usually the first clue that parasites are present.

(f) To identify the parasites, switch your microscope as necessary to medium (objective 25x) or high (objective 40x) magnification.

(g) Similarly you can mount any small parasite you collect from a fish on a microscope slide in a drop of water, to examine it in more detail.

(h) To help you identify to which major group a parasite belongs, use the simple key given below. Start from the top and take the choice that most closely fits the organism you see. Proceed step by step through the key until you reach the name of one of the six major groups described above in paragraph 9. Confirm your identification with the pictures and the location of the parasite on the fish.

Identification key to major groups of external fish parasites
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261b.GIF (41500 byte)

Using chemicals to treat your fish

17. Once you have accurately identified the agent, you should decide whether or not to treat the fish with chemicals. Ask yourself the following questions before making a final decision.

(a) Is a chemical treatment warranted? Are the fish seriously at risk from the parasite, and are there alternate approaches such as improving environmental conditions and removing seriously affected fish?

(b) Is it economically feasible, given the cost of the chemical, the value of the fish and the risks involved?

(c) Are the fish still strong enough to survive treatment?

(d) Is treatment possible in the presence of complicating factors such as an algal bloom, extreme water temperature, low dissolved oxygen or, for example, cloud cover which reduces oxygen production through photosynthesis in the pond (see Section 2.5)?

(e) Will the chemicals affect any other pond?

18. If you decide to treat your fish proceed as described next.

Organizing a treatment

19. Before initiating any treatment make sure that you have accurately identified the parasite responsible.

20. Then check water quality thoroughly, in particular the temperature, dissolved oxygen, pH (see Sections 2.2, 2.4 and 2.5) and total alkalinity (see Section 5.0).Increase water flow, if necessary.

21. Select the most appropriate curative treatment according to:

22. Carefully determine the total amount of chemicals required for treatment (see Section 15.0). Double check on your calculations, preferably with the help of somebody else.

23. Check the condition of the gills of the fish to be treated. If they have excessive mucus cover and/or are badly formed or chubbed together, the fish may be more susceptible to low oxygen levels.

24. Always have some additional means of oxygenation standing by, and whenever possible, stop feeding your diseased fish for at least 24 hours immediately before treatment.

25. Test the selected chemical and its dosage on a small quantity of fish, using exactly the same procedure as for the planned treatment. Observe the effects and after-effects carefully. If necessary modify the treatment accordingly.

Note: pre-testing becomes even more essential if you plan to use a chemical or to treat a fish species you are not familiar with (see paragraphs 28 and 29 below for the procedure).

26. During the treatment, make sure of the following conditions.

(a) The water temperature is as low as possible. Start the treatment either early in the morning (as long as dissolved oxygen is sufficient) or late in the afternoon.

(b) The dissolved oxygen content remains adequate. Provide additional oxygen as soon as necessary.

(c) The chemical is thoroughly mixed with the water mass being treated; dilute the chemical before use according to the kind of treatment. Apply it over the whole water surface area. Use a marker dye such as a few grains of malachite green to check more easily on the mixing of non-coloured chemicals.

(d) A close watch is kept on the fish being treated. At the first sign of distress such as coming to the water surface, gasping for air or erratic swimming, stop the treatment immediately and provide a good supply of well-aerated, clean water to the fish.

(e) Complete and accurate records are made, including chemical brand and batch number, dosage, water quality data, fish behaviour and losses, etc.

27. When the treatment is over follow the after-treatment care given here.

(a) On the next day, check a sample of fish for external parasites. Record your observations carefully.

(b) Continue to watch the fish for 24 hours. Observe and record possible after-effects of the treatment.

(c) Repeat the treatment only if it is absolutely necessary and not before two or three days after the previous treatment.

Pretesting a treatment

28. Whenever you use an unfamiliar treatment, involving either a new product or a different species, it is better if at all possible to organize preliminary tests to determine the relative toxicity of the chemical under your own conditions.

29. One simple way of doing this is as follows.

(a) Prepare a 5- to 10-litre stock solution of the chemical to be tested, according to the concentrations to be tested, as shown in the following chart. For example, if you want to test a new chemical within a range of concentrations 0.25 to 4 ppm active ingredient (Al), prepare a 100 ppm Al stock solution.

Preparing 10-l testing solutions from selected stock solutions
263.GIF (6385 byte)

(b) Prepare a series of identical plastic or glass containers of at least 10-l capacity each such as plastic buckets or glass aquaria, avoiding contact with metal:

(c) Add to each of the other five containers the exact volume of stock solution needed to produce the required chemical concentration in a total volume of 10 l (see the last column of the chart above).

(d) Add to each container the exact volume of clean water required to make the total volume up to 10 l. Mix well. The control is filled with 10 l of the same water.

(e) If possible add extra aeration to the containers using for example an air stone and air pump.

(f ) Add five of the fish to be tested to each container. They should be healthy fish, similar (stock, size, prior treatment) to each other and to the stock about to be treated.

(g) Carefully observe all fish during the test. Record if and at what time signs of distress are shown. Remove stressed fish promptly to clean and separate holding containers. Observe them until 12 to 24 hours after the normal treatment duration.

(h) Where possible, pursue the test for the normal duration of the future treatment, and then transfer the remaining batches of fish to containers filled with 10 l of clean fresh water. Observe them for another 12 to 24 hours.

(i ) Record the condition of all the fish at this point. From this, determine the maximum safe dosage of the chemical for the tested fish, the dosage at which all the fish in the batch tolerate the required treatment duration without subsequent ill effects.

(j ) If none of the dosages was tolerated, check the control treatment:

(k) If all the dosages were tolerated, repeat the trial with higher concentrations.

(l) The accuracy of this procedure will improve if you use replicates. Run this test with two or three containers for each batch of fish.

(m) Treat your diseased fish with a dosage at the most equal to the maximum safe dosage.

(n) If possible, test in a similar way, using infected fish, the effects of a range of safe dosages on the parasite responsible for the disease. Check the survival of parasites after the recommended treatment period. Then treat your fish with the lowest dosage which kills the parasites.


You wish to test the tolerance of 10- to 20-g juveniles of a new species of fish to a short bath of formalin under your own water quality conditions; proceed as follows.

(a) For a first series of tests, select a concentration range of 50 to 250 ppm formalin. Accordingly, prepare a 5000 ppm formalin stock solution by placing 50 ml formalin in a plastic bucket and filling it up to the 10-l mark.

(b) Clean well 12 plastic buckets, providing duplicate batches. In each of them accurately mark with a line the 10 l volume. Clearly number each bucket from 1 to 12 using a permanent marker.

(c) Add an exact volume of the formalin stock solution to each bucket, as follows:

265a.GIF (3655 byte)

(d) Add clean, well-aerated water to each bucket to fill it up exactly to the 10- l mark. Measure temperature in a few buckets.

(e) Install air stones and plastic tubing to provide extra aeration to each bucket.

(f) Add five juvenile fish of the new species to each bucket. Cover the buckets with fine netting to keep the fish from jumping out. Note the exact time at which the tests start, for example 07.40 h.

(g) Observe the fish for 30 minutes and carefully record distress (D) and removals, which can be summarized as follows:

Start: 07.40 h

(h) Move distressed fish in clean water to separate, clearly marked containers, until the end of the trial.

(i) Then, exchange solutions in each bucket with clean fresh water; move all tested fish back into their respective containers. Observe the fish for about 12 hours and note that some of the fish in buckets 4, 5, 10 and 11 are still not fully recovered by the end of the test at 20.00 h.

(j) Conclusion: the maximum safe dosage for a 30-minute bath is 100 ppm formalin for 10- to 20 g juveniles of the new fish species, in the conditions of the test, in particular water temperature ranging from 21 C to 22 C.

(k) Before treating all your fish infected with external parasites (for example Trichodina protozoa), run a similar test with a few diseased fish in formalin solutions ranging from 50 ppm to 100 ppm (for example 50, 60, 70, 80, 90 and 100 ppm solutions made from the 5 000 ppm stock solution). Determine which is the lowest safe concentration that eliminates the parasites in a 30-minute bath.

Choosing chemicals to control external parasites

30. All chemicals are not fully effective to control all external fish parasites or fungi. After identifying the parasite you must control, you should select the most appropriate chemical, according to the following chart.

31. If the best chemical is not available, a treatment may be attempted with some other one, as shown below.

Chemicals most effective to control
particular parasites or fungi

Note: for malachite green, see warning at beginning of chapter

Choosing treatment

32. Examples of treatments which have been successfully applied to farmed fish in various countries to control external parasites and fungi are summarized in Table 40. They involve the use of various chemicals:

33. Always remember that the chemical dosages and treatment durations given in Table 40 are guidelines only. You have already learned earlier (see Section 15.1) how the toxicity of chemicals for fish varies with water quality. But it may also change from one batch of chemicals to another:

34. Therefore, if you are not familiar with a batch of chemicals or a kind of fish, make a preliminary test before treating your fish stock (see paragraphs 28 and 29). In all cases, closely follow the procedures suggested earlier.

35. In addition to what you have learned previously about these chemicals (see Section 15.1), the following points are particularly important.

(a) Formalin can damage the gills of fish. Its toxicity greatly increases as the water temperature increases. In warm water you should prefer lower dosages of a mixture of formalin and malachite green. Make sure the dissolved oxygen is always above 5 mg/l during treatment, as formalin removes oxygen from water. Salmonids are less tolerant than other fish.

(b) Formalin and malachite green mixture can be prepared in two ways (Table 40):

(c) Malachite green quality often varies from one batch to another. Be sure you buy the zinc-free grade. Salmonids are more tolerant to this chemical than other fish. Chinese carp have little tolerance. There is a tendency to ban the use of this chemical in certain countries. Do not use on food fish.

(d) Organophosphates lose their effectiveness on parasites at water temperatures above 30 C. Salmonids are much more susceptible to them than are cyprinids or tilapias. There are two kinds of organophosphates (Table 40):

(e) Common salt: tolerance varies with species and age:  

(f) Copper sulphate toxicity greatly varies with water alkalinity. Do not use it if total alkalinity is less than 50 mg/l CaC03 (see Section 5.0).Remember that heavy rains may suddenly decrease the alkalinity of stream and pond waters. To treat fish in an earthen pond using a long bath, apply about 0.75 ppm copper sulphate for every 100 ppm CaC03 total alkalinity. Do not use if total alkalinity is above 400 ppm CaC03. Watch the effect of copper sulphate on phytoplankton and aquatic vegetation. Oxygen levels may be reduced.

Guidelines for the chemical treatment of fish for the control of external parasites1
(all dosages related to active ingredient concentration)

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1 Abbreviations: T = tank/trough; P = earthen pond
2 Qualities A, B, C and D are defined in the text (see paragraph 35)
3 Copp. sulph. = copper sulphate; Pot. per. = potassium permanganate; Benz. chlor. = benzalkonium chloride (see Section 15.1)