The relevance of fish nutrition has become increasingly important in the 21st century, largely due to the global increase in aquaculture production and the need for diets to meet growth at all stages of production from larvae to full grow-out and harvest. The onset of fish disease is often triggered when fish are stressed due to a variety of interplaying factors including poor diet management and suboptimal nutrition. The most likely cause of a nutritional deficiency in farmed fish will be a suboptimal level of an essential nutrient class such as one of the 10 essential amino acids, essential fatty acids or vitamins. Dietary imbalances of protein may arise from the presence of disproportionate levels of specific amino acids such as leucine/isoleucine antagonisms, and to a lesser extent arginine/lysine and cysteine/methionine antagonisms. Fish tissues contain relatively higher concentrations of highly unsaturated fatty acids than terrestrial mammals.