The importance of nutrition in certain areas, such as growth rate, and even to the overall survival of an animal, is obvious. Less obvious, however, is the effect that an animal's diet will play upon health and vigor in general. Many types of aquatic animals vary their diets throughout the year, in accordance with the demands imposed by their environments. Reproduction is, in particular, very much influenced by nutritional status, and it is the first function to cease when conditions are less than optimal. While some species may, in captivity, do well if offered a sound, unvarying diet throughout the year, others require certain types of foods or nutrients at specific times if they are to reproduce successfully.

Many of the maladies that plague aquatic animals can be directly linked to a poor diet. In some cases, the effect will be quite clear, where, for instance, a specialized feeder fails to adjust to a captive diet, refuses to eat, and dies. In other situations the link between poor nutrition and illness will be more difficult to assess. A deficiency in certain vitamins, minerals, or other nutrients can cause retention of body fluids, dull coloration, slowed or deformed growth, and a host of other conditions that may be difficult to link to dietary factors. Even less clear is the effect of diet upon a fish or an invertebrate's immune system. Animals that are able to survive on a suboptimal diet may nonetheless succumb to diseases or parasitic infestations that they might have resisted had they been fed properly.

When considering the requirements of the animals that you are interested in keeping in your home, bear in mind that certain species that may be offered for sale are nearly impossible to keep in captivity. Animals with highly specialized diets or those that will feed only when exacting environmental conditions are met are among these. Many of the most gorgeously colored and highly desirable of the marine angelfish feed, for example, almost entirely upon living coral polyps — a diet that is nearly impossible to provide for. Some of these demanding creatures may consume food items other than their natural diet, but such will not support them for long, and most perish rather quickly in the average home aquarium. In some cases this will be due to an inability to digest the foreign foods, while in others a lack of key nutrients will be the deciding factor. For example, seadragons, seahorses, pipefish, and related marine fish will all eagerly accept living brine shrimp in captivity, but, with the possible exception of the dwarf seahorse, Hippocampus zosterae, few will survive for long if dietary variety is not introduced.