Knowledge of fish nutrition is increasing but it has been mostly focused on commercial fish like salmon and not on specific fish held in cold or warm freshwater or seawater tanks. Pellets and flakes are available; some claim to be especially developed for specific species, but detailed nutritional information is not always available. Pellets fed in water should not be allowed to dissolve before eating to prevent pollution of the water.
Marine fish can be herbivorous, carnivorous, or omnivorous. Grazing or herbivorous fish eat plant materials from the rocks of the sea and need more fiber than carnivorous fish. This can be accomplished by feeding plant material in a basket in the water or by feeding an herbivorous fish pellet. Carnivorous fish should be fed a diet with high amounts of protein and fat, which can be a pellet or different fish species. Non pelleted food for fishes can consist of artemia, algae, squid, herring, mackerel, whiting, sprat, bream, and shrimp. It is important to know the origin of the food, because threatened fish or fish harvested in areas or ways that negatively affect the environment should not be fed. Moreover, the fish should not be contaminated with heavy metals or organic components such as PCB and DDT.
Vitamins should be added to fish diets, including vitamins E and B1 and stabilized vitamin C. Iodine should be added to prevent struma in sharks and rays. Some zoos also add glucans. Vitamins and minerals can be injected into the fed fish. Alternatively, tablets can be added just behind the gills of the fed fish.
Fish products or pellets should contain the right amount and type of feed. Regularly checking whether the fish are too fat or too thin is an important factor in proper feeding. In a mixed exhibit, the feeder should ensure that all fish get the right amount and type of food. If possible, sharks should be fed individually.
Also see Nutritional Diseases of Fish.