I get clicking out of my ritteri tank too. I'm *fairly* certain it doesn't have a mantis in there but what I do have is about .. probably around 12 or so crabs in there ranging in size from 1/4" to 1.5". So maybe you have a crab, not sure. I do lose more snails then I think I ought to be losing (AND the little buggers make sure I can't keep any corals in there, yum yum).
In my case the tank is 30" deep and I'm 5'9" so I can't really reach the bottom, so they see me coming, and a pest crab only seems to catch the dumb cleaner shrimp ("YUM FOOD YUM!! .. Oh.") My only hope for removal is tearing the rockwork apart which I can't do yet on account that touching or moving the anemone usually means a tankfull of dead fish. I'm having LOADS of fun with this tank these days. Hope your situation goes better than mine..
