It really needs to be in a shoal asap. They're very "highly strung" in small numbers.
Give it a good meal or two and get it a bit more rounded and it will look just like a Red Piranha apart from the head. Young of this size actually hide within shoals of similar sized Piranhas for protection. So do the young of fin-eating Serrasalmus piranhas. That would be one tricky shoal to keep happy, lol.