I seem to be missing a pretty big frag of zoas. I think I know what happened: every night, the pistol shrimp works on his burrows, and there are new piles of sand at the front of the tank. The zoas either got buried or fell into a sinkhole. I poked around in the sand at the front, and I can't find them anywhere! I moved all my other frags further from the burrow openings, so hopefully they won't disappear too. Stupid shrimp......