There are some commercial foods available for hermit crabs, although none is complete according to many seasoned crabbers. Hermit crabs need to be fed certain types of supplements in addition to the commercial foods. Crabs need more protein than is provided in those foods, they need to feed on shrimp, tubifex worms, and algae wafers (all in the fish supplies section). They can also be fed cooked, unseasoned meat to help meet their protein requirements. Just do not leave it in the cage longer than a couple of days. Hermit crabs also require calcium for good exoskeleton growth. Good sources of calcium are clean, dry eggshell pieces, or cuttlebone (for birds). It is also a good idea to feed the hermit crabs fresh fruits and vegetables on a regular basis. In the wild, the crabs are scavengers and eat a variety of foods to keep healthy.
Water is another key component to healthy and happy crabs. Hermit crabs require fresh water at all time; they also need saltwater provided to them. Some species of hermit crabs need saltwater for survival, the other species like to soak in it occasionally. Since most of the places selling hermit crabs will not know what species they are dealing with, it is highly recommended saltwater be offered constantly along with the fresh water. When mixing the saltwater, use a salt made for marine aquarium fish. Do not use table salt it is poisonous to the crabs. The easiest way to offer the two types of water is in plastic or glass dishes, deep enough for the crabs to submerse themselves, shallow enough to get out without help. Placing a piece of natural sponge in the water will help smaller crabs get out while still offering enough water for larger crabs to soak in.