wow. Here in MO i got rid of several piranhas, bought them as babies for about $15-20 then sold them for more as i weeded them out cuz they were bigger. I probably would have done the same thing even if i had to give them away for free as i weeded them out simply because of how much stronger, healthier, and more beautiful of a shoal i got doing it that way (also saves you from the other piranhas eating the ones they dont think belong lol)
As always its not the only way to do it, just what i recommend from my experiences with them.