by monsters i just meant something that would be aggresive and predatory
If you want something impressive, deciding on a large centerpiece and planning around it might be the best way to go. Check out some cichlids, cruising catfish (lancers, pims etc.), other predators like wolf fish, snakeheads if you're outside the US, predatory characins (true gars are a no-go at this volume but this group has plenty of gar-like fish) and piranhas, and pick one you like. Then you can check what it's compatible with, or if it needs to be a solitary animal.
Cichlids are your best bet for a true community. They coexist in relative harmony with many tough, armored catfish native to the area (larger plecos, doradids, driftwood cats etc.), so you can set something up that matches what your "main" fish would face in its native habitat, and hopefully not have too much aggression. Of course, cichlids are constitutive sociopaths so you might also wake to a bloodbath.