Hello. I had one of these crabs for several months.
First of all, you need to give them a half land/half water setup. They will spend a lot of time out of the water. I wouldn't worry too much about them catching fish. They are really slow moving critters. However, they will eat other crustaceans. Mine loved to chow down on local crayfish.
Do some more research into long term care. I think mine died because i didn't provide with brackish water and a very limited amount of iodine. Bigger crustaceans like this do require small doses of iodine from time to time.