Not sure about red light, but I've had similiar problems. I used a maglite with a red lense, some fine point tweezers (used as spear to get crab out of hole), fine netting (mono) and some bait. I put the bait inside a ball of mesh netting, near the last known location of crab. Wait several hours (mine usually came out 2-3 hours after lights out), check mesh quickly with light. Know the exact location of mesh, don't scan whole tank with light, the crabs seem to detect some of the red light (shadow maybe?). If crab flees to hole, use tweezers to capture. It took me a week, once I started using this method (2 months of trying other ways 1st). Good luck.
The crabs in my tank moved really fast, but not far. Usually they just went around a corner, to a crevice. After all was said and done, I figured out that if you placed a rock near the trap, they'd run to the crevice under the rock if you spooked them.
nice advice O Pi Ing :
"How about buying/DIYing crab trap to catch them instead?"
Ever bought a aquarium crab trap, let alone ever found a 2" crab trap? We're not talking mantis shrimp, so mantis traps won't work. Fish traps available to hobby don't seem to work either.
