Im not a fan of piranhas, sorry if that offends anyone. I find they just sit there and wait for food. I would prefer action when there isnt food in the water.
lol well if your ever interested just pm me. i can get them close to feeder fish price so ill ony loose a liltle bit of money (bout 10 feeders worth) so i dont really care.
The only preditor that i can think of that is always moving, is an arowana. Most are pretty still when food is not present. I don't think you can keep an arowana for long in a 90-100 gallon.