I will be collecting my crabs on the beach in miami, will there be any problems with this? Also, in case of an emergency (ran out of crabs and i cant get to the beach) is it ok to buy crabs that are sold as bait at fishing store? Or are these treated with some sort of chemical?
Live crabs are inverts and are quite sensitive to chemicals. I've never heard of live crabs in a bait store being treated with chemicals. I believe others on this site have purchased crabs from bait stores.
If you put them in your saltwater tank, the'll live a few weeks. If you give them a little habitat in a low plastic container with sand and a brackish "pond", they can live for years!
Feed him something every day. Give him a crab one day. See if he will take a piece of thawed frozen shrimp the second day. (I'm assuming these crabs are farily small).
Watch to make sure he eats the whole thing within an hour or two, then remove the uneaten food. You can adjust the food - if some is uneaten, give him less the next time - although I know that's difficult with crabs!