hey,
If they are together, they need to be red belly piranhas, or else they would kill each other, in most case.
The general calculation is 20galls per fish, so u need a 120galls water tank.
They need place to hide, as they fight a lot and would some time also tend to eat the smallest or the weakest one.
They should be real good filtration, as they eat meat, fishes, in shot...any thing that moves and has meat, so the left-overs would pollute the tank.
Also make sure there is good current in the tank, to keep them active.
Most important, have a good study temperature, as they are tropical fish.