Don't like the 55 gal limitation. ..... Otherwise you can have a tiger fish. Mean and super aggressive.
But you still can have small species of black piranha, they mean and need to be kept alone, you can check serrasalmus genre piranha, some of them can fit in a 55 gal and them all mean.
I had a 1 inch sheepshead minnow (Cyprinodon variegatus) that killed some small sunfish, a blue gourami and many goldfish including a fat 5 incher. It was absolutely relentless. The ones that were athletic enough to jump 3 inches over the lip died of dehydration, the ones that couldn't were ripped apart.