The thing Raven didn't mention is that you might not know you have a mantis for a long time after adding the new rock. They are shy buggers, and if they're in a tank with active fish, they won't come out in daylight. Your first clue will be mysterious missing fish, and maybe clicking noises at night.
There are several different kinds of mantis shrimps. The 2 major types are smashers vrs spearers. One has a large, clunky claw that is used to club it's meals (or pesky aquariasts). The spearers have claws like ginsu knives--they can hit hard and fast, and slice their meals (or threatening fingers). Colors vary greatly, from bright lime green to the harlequin's multi-colors. Most max out around 5-6 inches, but will be MUCH smaller when introduced--anywhere from 1/2 inch up.
I do agree they are cool critters. These annoying buggers have one of the most complex eyes of all animals, and rank right up there in brain power for inverts. Having said that--trapping them isn't a good method, because you get one shot. If it fails, they've learned how to avoid/defeat the trap, and you'll have to try something else. The FW method works--sometimes. If the mantis has a deep enough hole, you might not be able to get enough FW in there to make it leave.
