Piranhas are very messy eaters as you already know by now, what was said regarding the 50% wc on a weekly basis may work right now that they are small but as they get bigger they will need much more space and more frequent water change in order to stay healthy. As your water deteriorates, the fish will slowly get sick and the others will pick them off one by one. Often when the sick one is attacked by the others the defending piranha will damage the attackers, which creates the next one that will get taken out.
If you keep water clean feed healthy foods and give them ample space to get away from each other and have a place to hide, you could have a nic pack of deadly but beautiful red bellies there. If you don’t do the wc they will grow even slower than usual, which is pretty slow once they hit about 3-4”. As a general rule when I had them lfs would tell me that you need roughly 25 gal min per fish. Knowing what I know now that is still not very much considering the fish’s needs.
Hope it pulls through for you.
Ps if you look in the forums there is a place dedicated to piranhas that folks there can help you out with more specifics on these guys and who also share a love for these guys. Hope this is helpful.