Fish generally require a high-protein diet, with a significant percentage of protein provided by fish meal. Most fish do not synthesize ascorbic acid; therefore, adequate supplementation is necessary. Ascorbic acid should be provided in a stabilized form, often referred to as “stay C.” Proper storage of feeds in a cool, dry place is necessary to minimize nutritional degradation.
Classic ascorbic acid deficiency has been referred to as “broken back disease” (scoliosis) by fish farmers because of the collapse of the vertebral column that occurs secondary to inadequate collagen synthesis. Less dramatic signs of deficiency include impaired wound healing and immune function. Deformation of gill cartilage, visible on wet mounts of gill tissue, may indicate ascorbic acid deficiency.
Of the B vitamins, folic acid deficiency has been associated with poor growth and, in channel catfish, with anemia. This disorder has been referred to as nutritional anemia of catfish, and is caused by bacterial contamination of feed. If nutritional anemia is suspected the feed should be changed to a new bag from a different source immediately. Pantothenic acid deficiency has been associated with clubbed gills, also referred to as nutritional gill disease. Acute thiamine deficiency has been associated with neurologic signs, including convulsions and death; chronic deficiency has resulted in loss of equilibrium, edema, and poor growth. Riboflavin deficiency has been associated with vascularization of the cornea, hyperpigmentation, clouding, and hemorrhage of the eyes. Niacin, biotin, and pyridoxine deficiencies have been associated with neurologic abnormalities, including spasms and convulsions. Choline and inositol deficiency have been linked to poor growth. Vitamin A deficiency has been associated with poor growth and retinal atrophy. Vitamin E deficiency has been related to myopathy, including muscular deformities. Skeletal muscle abnormalities have been associated with selenium deficiency and rancid feeds. Rancid feeds have also been linked to steatitis. The nutritional requirements for trout, salmon, catfish, warmwater fishes, and aquarium fishes have been published by the National Academy of Sciences.
Improperly stored feeds may be subject to contamination by mycotoxins. Aflatoxins, produced by fungal contamination with Aspergillus spp, causes hepatomegaly and hepatocellular carcinoma in rainbow trout. Fish feeds should be replaced frequently; for dry foods, every 6 mo, for wet or semimoist foods, every 1–2 mo.
Feeding methods and rates vary with species, age, system, and water temperature. A maintenance diet is generally considered to be 1–2% of body wt/day, whereas a diet for growth may be 3–5% of body wt/day. Some ornamental species may require more than this. Feeding behavior can be an important indicator of health. Vigorous feeding is desirable, and a sudden decrease in appetite should be cause for alarm. To minimize size variation within a population, feed should be distributed over as broad an area as possible. Smaller, more frequent feedings are preferable to one large feeding, especially for young fish. Professional aquarists should be encouraged to periodically weigh valuable fish, keep an estimated weight for total fish biomass in individual tanks or exhibits, and weigh feed for these animals.