If you had a mantis or a pistol, you would probably never see it, unless you were specifically looking for it. You would however, hear clicking from the tank. The nems could be the culprit but if you feed them they will usually not bother your livestock. Do you feed the nems? Have you lost any fish, or are you just speaking of the experience your friend who gave them to you had? If you are still losing fish, then I suggest you post specifics, ie. type of fish, water parameters, etc. and we'll help you figure it out.