Its not advisable to keep crabs with fish, if they get hold of any fish, they will without a thought, kill them. I believe they are fed earth worms and crickets but will also eat algae and certain moss. They require an aquaterrarium to live well and it has to be kept very humid. Its a bit of a specialist care sheet for them and to be honest, since they only feed at dusk and dawn, its not likely you will see much activity. Maybe just start with your fish, get the hang of it and keep it as an option for later on when you've got a wealth of knowledge in your head.