I had the same experience with my Pygmy, he re-arranged the tank to his liking, but never hurt any of the corals. He left my fish alone, but loved to feast on emerald crabs. It was funny to watch him pull snails off of the glass and chuck'em around the tank or try to get hermit crabs out of their shells. I put a cleaner shrimp in the tank with him and he'd get soooo frustrated when he couldn't catch it. I didn't even try sealing my tank considering he could squeeze thru an opening the diameter of air line tubing, so I'd catch him chillin out on top of my lightstrip at night. I decided to get rid of him when he learned a new game, "pull-the-heater-off-the-wall-and-whack-it-against-the-glass-until-I-came-running". I'm surprised he never shattered the heater, frying himself and his tankmates. His name was Sparky and I still miss the little devil.