What about a pair of some larger species....such as Jags...Festa or you could maybe try a mix like a male Jag and Red Devil or a similar combo. That might be luck if they get along though. I like pairs myself...less problems in smaller tanks like 125.
I'd go with a shoal of piranha. Like import racer said. Pick up a dozen little guys and see were it goes from there. They don't get too big and they look cool in a group. Good luck with your decision.