Mantis shrimp are pound for pound one of the most awesome predators of our world. With 4-D vision, they are able to create movements so precise as to kill a fish in one strike. Depending on the type of mantis shrimp (spearing or striking) they are capable of making a strike comparable to a 22 caliber bullet, and also known for cracking tanks if the strike is done at the tank wall. As for them killing any fish in your tank, I have heard of accounts of the Mantis Shrimp killing up to 9 fish in one night. Obviously these are extreme circumstances, however don't take this predator lightly. Ways for identifying this predator and getting rid of it in a humane way can be found by doing searches on google.com for Mantis Shrimp. I loathe at the capabilities of this predator, however don't want one in my tank.
Good Luck!