The two most aggressive fish that i have ever kept were the peruivan Black Romb and a Serr. Gibbus otherwise known as a "gold". Both were true bastard fish....killed numerous tank mates over night. I mean they wouild have like 40 or 50 feeders in the tank and 150 gallons to work with and still pick on the reds or other smaller black. Dont get me wrong i was new to p's at the time like five years ago but still they were true finger chasers and true killers. I tried to give my black romb to three different people and they would end up calling me days later and negging me to pick him up because he was on the verge of killing one of their fish. I always made a joke that if i found a underground piranha fighting rink i would put a heavy wager on his ass killing any other piranha out there. I still have both of them because the fish store told me that they didnt want the liability of it killing anything and told me to put em down. But i didnt have the heart i just kept them as solo fish. if you want my gold Good luck he kills anything he stays with..... hes yours