How big is it? The bottle method may work. Take like a coke bottle thats clean. Cut the top area off and turn it so its facing in. Staple it or super glue it. Poke some holes in it so water will pass through. Put some kinda food in it, and put it in your tank where you think the crab is. The only down to this method is you may catch a lot of other critters before you catch the crab.
hth
-Josh