I was thinking just to help fertilize the plants in there. I know any piranha makes a heavy bio load so as much as can be done will help. I was thinking a shoal of about 6 and they can mow back some food. Im even toying with the idea of neons in with them. I have heard of varying degrees of success with that. The RBPs at my LFS here are only about 2 inches long and i know they grow fast. Hopefully the neons will be faster
I think they will be happy with a 125 or a 220 gal system.