Some of the big hermits will try to grab or pounce on anything, especially at night when they are most active. I've had some pretty large ones with blennies and ocellaris clowns without trouble. I knew a guy that brought one back from the beach at Corpus. Big mistake that was. Injury leads to trouble and the crab wins, even over a large puffer eventually. An eating fish is not necessarily ok unless its deficating correctly too. I have a club anemone in one tank. It lives within a hole or cave in the LR, only venturing out at night. Its sting is toxic and very deadly. If a fish gets stung, the fish is crab food after the anemone gets done with it. There can be all kinds of stuff in LR that rears its head just evry now and then, say on the full moon? Its really hard to buy a 1.5 inch tang killer hermit crab, even if it did really happen. Do you know what kind of crab it is Jenn? What LFS would sell you non-reef-safe crabs for your tank? If the LFS is picking them up in Corpus, those critters are bad dudes in some tanks.