Making a piranha tank more lively?

What could I do to liven up my piranha tank? I have 4 red belly piranhas. they are about the size of my hand or so...6 inches or a little more. I have two over top filters that produce current. aswell as a power head that creates current. back then this worked in getting the piaranhas active and swimming....but I guess now that they are bigger this doesnt faze them. they all just gather on one side of the aquarium now and will only move around when i put food in to feed them. the tank temp is around 85-87 degrees. would introducing more piranhas make the tank more lively??