Dealing with a predator in a reef

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If you're pretty sure it's a mantis or crab then if you don't have a lot of corals encrusted on your rock you could try dipping the rock, either a hypersalinity dip or a brief FW dip, especially if you have a good idea which rock the critter is hiding in.

I've had a clicking in the back of my tank since I set the tank up 4 or 5 years ago, never lost any fish to it and never saw any mantis. In the first year I trapped out 10 or 15 crabs but whatever is making the clicking never went into the trap.