i agree with chris. also, check your hermits closely for type - the only ones that i have found to be totally reef safe (except for killing snails) are the blue legs and the scarlet legs. if they have black legs or black and white legs, they tend to be more destructive. also, you may have something there that you dont know about coming out at night, like a pencil urchin or rock boring urchin.

i have also heard that sally lightfoot crabs will turn a little more carnivorous when they get large