Some possibilities:
American cichlids:
Convict
Firemouth
Neet
Apistogramma
Ram
Angelfish
Curviceps/ other keyhole dwarves
MAYBE Salvini, but I'm skeptical about that.
Anything 6" or under, really....
Africans:
Hemichromis Jewels ( Lilifali, bimaculatus )
Kribensis
Buffalo Head
Teleogramma
Most small mbuna, 4-6" variety ( Many species there. )
Shellies ( Neolamprologus Meleagris, Brevis, etc. )
Lelupi, brichardi, most any julidochromis
.....there are actually quite a few options. You may not be able to keep alot of fish, but you do have options.
Like I said, anything 6" or under is perfectly fine, assuming you set up the tank right and mix species correctly if at all.
-Zeb
edit: Oh, I'm sorry. I overlooked the fact that you wanted "aggressive". Of course, that can depend on what you think is aggressive. They all CAN be, with the exclusion of rams.