i would suggest, if you plan to set up a community tank would be to get some fish from the same part of the river as the piranha you have, those types of fish are typicaly built to withstand these problems in the wild, of course that would only work if you have a very large tank.
I had thrown a bunch of rosey red minnows into my RBP tank as feeders. They ate all except the one that was gray/ blackish in color. Wierd. It survived numerous feedings being left alone for weeks.
Just when I thought there was some kind of bond between this dark minnow and the RBP's, I woke up to find it GONE!
okay well here's another question, for those who have Piranha.
How long does it take Red Belly Piranha to grow 1" bigger?
like my Oscar it doesn't even take a month to grow 1" bigger, 2 of my Piranha have been the same size since I got them in December from someone else, I just recently got the 3rd and smallest but most aggressive of the 3
I also had a 2-month blue guarami(SP) I was convinced wouldn't ever leave the tank.
He was swift and made a home beneath the Emp400's overhang.
Evaporation happened, and one day he disappeared.