I like mine too. Yes, they can knock stuff off the rock if it isn't nailed down. It will also pick up small frags to use for camouflage, but not if the frag is attached to rock. Mine is sporting three small Zoas right now. They eat coralline algae, so if you're a fan of real purple rocks, the Urchin slows down that process. I just think they're a very interesting invert and completely reef safe.