...black nasty's are EXTREMELY aggressive and territorial!
I have a 6 inch male Convict, with orange on its belly and blue on its fins, beautiful convict, mean as ****. It ripped the eyes out of a 12 inch oscar in 10 minutes after the lights went off, I knew because I heard a large splash and wham, there was my oscar dead as a door knob.
The most agressive cichlid I've ever seen was a ~28 inch Dovii, this MFK guy has videos of it all over youtube, and the thing is just nasty.