I had a 1 and half inch convict keep a 3-4" piranha in the corner of my tank until i took the convict out. it was the convicts tank and had been for 2months and i added the p, because i had a tank crack. this lasted for a few hours until i could run to the lfs to get a new tank. you have to love the fish to really understand what they are all about. if you think you will see what you have seen in the moives then you will be very pissed.
I have a black pirahna. I've had him since he was about the size of a qaurter. He is 5-6 inches long now. He will not eat in front of anyone but I also don't starve him. You could spin around in front of him and he will eat whilwe your back is turned. If you want a frenzy feeding with most any pirahna you need to have like 20 of them together and feed them on a regular basis or they will start to eat each other.