Very true, your solo hunters are alot more brave than your pack piranhas. If you are interested in the piranha family some big rhoms I have seen are aggressive, cichlids like dovii and umbee are great predators and fearless, Fahaka puffers are fearless as well, Wolf Fish are a little less active but very aggressive predators nonetheless, armatus, and bowfin can be very aggressive. These are jus some to help you out, but any one of these would slaughter a piranha one on one. Movies have really hyped Piranhas up more than what they are, dont get me wrong...all fish are different and their are some bad ass ones but for the most part their prey is weak, injured, sick animals that are near death. Hope this helps....
Sevendreamz