Removing crabs and mantis from a reef aquarium.

Purchase a length of timber dowel from a hardware store and cut it to around two feet, or definitely the depth of your aquarium.
Use a very fine drill bit and drill a hole in the end of it, get a two inch nail or a section of stainless steel rod and make it really sharp or a large needle and force it in to that hole.
Hover above the crab and push hard and quick to harpoon the crab, all it needs is the major shell area with a hole put in it and the bacteria that get in will kill it.

As for a trap, get a small plastic soft drink or mineral water bottle and cut the nozzle off and turn it around and put three holes to line up on bottle and nozzle and tie the nozzle in place with fishing line so it can be undone later on.
Put some holes on the bottle so the crab can find the bait with in by smell, I use a soldering iron for that sort of thing in relation to the holes.
Tie your opened muscle bought from woollies or a bit of prawn in the centre so nothing can reach it from out side of the trap.
Place some araldite or silicon in the nozzle and sprinkle some grit on the glue so the crab can get easy traction to get into the trap and you have a trap to use each time you think you have a mantis or crab that quite often comes in live rock and grows so that one day its big enough for to see it and can kill your tank life.
I weight these down by tying bits of dead coral on the sides so it stays put.

“Never” use a trap when lights are on or you will catch other life that has a place in your aquarium.
Place trap in tank out of strong current as lights go off and take it out as lights comeback on each time.
Or just buy a trap from the lfs.