greatfulxdead;2993211; said:
for serrasalmus, yes they are solo species, except Geryi. However, they have been found in the wild in small or loose groups, but no way for most of us home hobbyist to duplicate the wild at home. (if I can get a multi-thousand gallon tank I'd would so try a cohab)
if you go with pygocentrus then they can be kept in a shoal, provide that you have a large enough tank for the amount of fish you'll be keeping. fin nipping will still happen as that's in their nature but as long as you have enough space, keep them well fed and good water parameter, fin nipping should be minimized. However, some pygos are more cannablistic then other. Piraya being the most cannablistic and caribe is second runner up then the common red belly. it's really up to the individual piranhas and the condition they are kept in. there are few RIP here and there but there are still way more successful happy ending compare to the RIPs.
mines suppost to be a gold diamond he is about 6 right now lil under maybe 
but i say give me some amount of years