Click click

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That is the sound in my tank the past few weeks. Usually one or two clicks at a time.

Some very live rock was introduced about 3 months ago. I have trapped and removed 1/2 dozen crabs and there are still more in there.

I haven't seen any mantis shrimp or pistol shrimp although I've looked and looked.

Is it possible the crabs could be making the clicking sound? Is the best approach to watch and wait, remove the rock and go on an aggressive mantis hunt, or continue to set traps?