Publisher Summary It is usually extremely difficult, under farm conditions, to define nutritional diseases in absolute terms, because it is rare for a single deficiency of one essential nutrient to exist. Nutritionally compromised diets will often render fish more susceptible to infectious conditions. Although most deficiency diseases are associated with a complexity of marginal or absolute deficiencies, it is only by detailed experimental studies of single-deficiency conditions for each species that a true understanding of nutritional pathologies of fishes can be obtained. Deficiency diseases are of two types, deficiency or imbalance of the macronutrients in the diet—the protein, carbohydrate, lipid, and fiber—and deficiency of the micronutrients—the vitamins and minerals. Although almost all necessary minerals are available in most practical fish diets and fish can also absorb minerals from the surrounding water, mineral deficiencies do, on occasion, arise in farmed fish. The reduced bioavailability may be associated with a dietary imbalance or with presentation of the element in an unsuitable form. It may also be associated with interaction with other dietary ingredients, such as vitamins and fibers. Heavy metal contamination of feeds resulting from storage vessel leaching or from use of unusual feed sources can cause significant growth inhibition. Probably, the most important of the natural contaminants are the mycotoxins, metabolic products of fungal contaminants of feed components. Other toxic agents include cottonseeds senecio alkaloids, anthropogenic chemicals, and photosensitizers.