Aggression can be defined several ways. Exodons are very aggressive in a shoal. Also called the bucktoothed tetra, they're a small minnow shaped tetra with teath, another denzien of the Amazon. I've kept them before and they will harass any other fish in their tank to death unless that fish can kill them. However they don't actually eat other fish. Their scale eaters, picking at other fish and often blinding them. In sufficent numbers they will put on a show ripping up earthworms of which there are videios in youtube.
Other options might be some cichlids. I'm not a big cichlid person so I havent owned many personaly but my dad still claims his jewl cichlid from when he was young was meanist thing he'd ever seem. I dont know where jewls come from, african rivers I think. Some of the rift lake remain small as well so they might be an option you'll have to do some reaserch on the individual ones.