My buddy has 4 red bellies in a 55 gallon.
and has a Serrasalmus manueli (biggest species of piranha known) juvinile in a 30 gallon. which is moving to a 125. since they get about 2-3'
Personally, I really dont think piranhas are worth it. they aren't very active fish. They just bob there, until they are fed. and even during feeding, by no means is it the frenzy one would expect. just casual nips for a while until the prey is gone.
they have the reputation though, which is 80% false. they are some of the most skittish fish i've seen. You walk in the room, and they bash into the glass on the other side of the tank trying to escape.
I'd have to say if your really looking to get an agressive fish, get a flowerhorn, or a snakehead. snakeheads have the power. and flowerhorns? **** I wouldnt want to be 20ft near a flower horn in a pond.
I'd have to say piranhas are mainly cool for the reason that you can go around and tell people HEY MAN I GOT PIRANHAS! and they'll be like **** DUDE! WHOA!. If you starve em for a week, they'll put on a good show though.
my 2 cents.