How bad are these things? I've had one who hitched a ride on some live rock and has been in my aquarium since the first tank. He's managed to live through some pretty tough times. I actually thought he didn't survive the amonia spike a few months back but I spotted him the other night, and he's getting really big! What do these guys eat? I don't see any destruction in the tank but I only have a few corals at the moment. What couldn't I put in the tank if I decided to leave him there?