An enormous variety of frozen food items suitable for use with marine and freshwater fish and invertebrates are readily available in most pet stores.
Among the many that you may find are plankton, krill, prawn, algae, fish of several species, clams and other mollusks, crab eggs, and squid. Pay particular attention to the various combination diets that include a number of ingredients, as these may more closely approximate a balanced diet for some species.
Frozen foods sold for feeding aquarium animals are usually exposed to gamma rays to kill potentially harmful parasites and other microorganisms. This irradiation may remove certain vitamins and trace elements from the food. Therefore, you may wish to soak frozen food items in a liquid vitamin preparation before feeding them to your pets.
Where legal, you may wish to collect your own food items and to freeze them for future use. Freshwater and marine habitats will yield an incredible wealth of suitable food animals such as small fish, crabs, insects, shrimp, amphipods, and the like, as well as numerous types of aquatic plants and algae. See the cautions mentioned elsewhere in this chapter concerning the possible transmission of disease organisms that may occur when using wild caught food animals.