It is important for farmers to recognise at least the most common nutritive deficiency symptoms. Deficiency signs of farmed tilapia may occur when fish are fed nutrient deficient diets or raised in a low nutrient-input culture system. Essential amino acid (EAA) deficiency in tilapia generally leads to loss of appetite, retarded growth, and poor feed utilization efficiency (Table 27, EAA/EFA). In some fish species (e.g. rainbow trout, sockeye salmon, Atlantic salmon, chum salmon, coho salmon), lysine, methionine or tryptophan deficiency results in various signs such as scoliosis, lordosis, fin erosions and cataracts although none of these deficiency signs have been reported for tilapias. Similar to EAA deficiency, the lack of essential fatty acids (EFA) will also lead to loss of appetite and poor growth in tilapia. Other reported signs of EFA deficiencies in Nile tilapia include swollen pale and fatty livers.
Mineral deficiencies are difficult assess in tilapia as most trace elements are obtained both from the dietary ingredients and from the culture water (Table 27, minerals). The following deficiency signs have been reported for Nile tilapia: calcium- reduced growth, poor feed conversion and bone mineralization; magnesium- whole-body hypercalcinosis; and manganese- reduced growth and skeletal abnormalities. In a study by Dabrowska et al. (1989) with Nile tilapia, excess magnesium (0.32 percent) in a low-protein (24 percent) diet produced severe growth retardation and showed a significant decrease in blood parameters, haematocrit and haemoglobin content, and magnesium deficiency in a high-protein (44 percent) diet caused whole-body hypercalcinosis. A dietary magnesium content of 0.059-0.077 percent was adequate for optimum performance of this species.