+1
Night is the only time they can catch fish so make sure you let the tank get nice and quiet and the fish are in their suspended animation state (fish don't sleep more on that in FISH 314, Fish Physiology). ANYWAY I think its time to invest in a mantis trap, those clear acrylic tubes with trap doors. Just because you can't find the mantis doesn't mean it's not there, it has ALL night to hunt and it may just decide to hide out all night on the one night you decide to look for it(see murphy's law)...good luck.