Sometimes you will find a rogue crab in your setup that needs to be removed. Depending on the type of crab you encounter you will want to try different methods if catching him by hand doesn't work. For swimming crabs, use a fish trap, we have one outlined here. For crabs that rarely swim, or cannot swim this trap will work:
Take a small square or similarly shaped plastic object that is free of any chemicals and has been rinsed thoroughly. The Glad company makes a disposable product that is suitable for the job. Remove the lid to the tupper, and place the tupper in the bottom of your tank, preferable in an area the crab is known to hide. Then place the lid in the tank and position it as a ramp that leads up to the top of the tupper, place bait inside (anything meaty should do the trick, sinking pellets that are hard an uninteresting to your fish will produce the best results though). Hopefully the crab will crawl up the ramp, and into the tupper to eat the bait left for him. But since there is no ramp to use as an exit they remained trapped inside the tupper until you can remove them. For swimming crabs use the trap outlined here.