Serrasalmus Marginatus

[moeplz]

some serra can be kept together like spilo's or macs and geryi but its an uneasy truce and needs to be a decent sized tank otherwise they fight and end up looking like crap.
i agree with kido though serras are generally a solitary specie of any kind.

Yeah...although some people have kept some serras together, its a very risky investment unless you have a very large tank. If you want more than one piranha than I would suggest getting some pygos.