I check out my tanks at night quite a bit and can hear an occasional loud snap or pop. I thought it might be a mantis, but I've never seen it in action. I've lost a couple of small fish over 8 months, but still have a lawnmower blenny 2", several peppermint shrimp, emerald and hermit crabs. I think they would all be toast by now. I'm guessing it's a pistol shrimp, but I can't see it either. Anything else go snap that I have mentioned......... Looked for burrows, mounds, debris. Have seen nothing that tells me one way or other. :?