Is the tank well planted? Piranha feel much safer and are thus calmer when they have plants to retreat to, especially when young. The better they feel like they can hide the less need they'll feel to hide. Other things that might help are: if you have bright lights dim them, try peat in the filter or black-water extract to make the water just a little murky and closer to their natural habitat in the amazon. I'd recommend not giving up Piranha for me have been very rewarding fish, more so when their older. They become very impressive fish. But, they are a little a 'twitchy' for lack of a better word. The bigger they get they'll become more confidant and this should diminish, but probably never completely. Its part of their charm. They live in one if not the worlds most dangerous rivers, filled with aquatic horrors of every kind. If you lived there you'd be a little paranoid too.