Day 2 of cycling my 4 ft new tank with live rocks and discovered a good sized hairy crab inside. I saw it when I switched on the lights in the early morning. Mood half gone as I do not want to redo the rock work to catch the crab. So I set up a trap with bait and hope for the best. The crab fell for it on the third day after setting the trap. I put a piece of market prawn in a clear plastic container and pour in some sand to weigh it down. Then I place the container at an angle with opening resting on the piece of live rock next to the crab's hideout. Once inside the container, the crab is unable to climb out due to the slanted smooth surface. This trick was shared by some websites and it worked for me.