It is best to have half water and half sand.

Crabs are omnivorous.

You can feed them fish, ah, either raw or dried, they can take it. You may also feed them fish feed, red worms, shrimp, snails, mussels. It's best to feed them some grasses regularly, bitterweed, ilex algae, etc. As long as the feeding is ok,you don't need to worry about the rest. Crabs are very good to keep.