Beani. I have never seen anything like what I did a year ago. Had just got a 1 inch beani. Put him in a 30G long with a 1.5 inch RTM. They immediatly didnt like eachother in a big way. Beani went straight for him. I immediatly seperated the tank with a divider. Afterwards, I went and got my neighbour to show him my new fish. Somehow the beani had jumped the divider and in the 15 minutes i was away they had shredded eachother. They both had almost no tail at all, and even on their tiny bodies i could tell they didnt have a lot of scales left. I seperated them again and we watched them for a while. Everything seemed fine and we left. When i returned, the beani was once again on the RTM's side of the divider. Dead. The RTM lived for a day after and then died with no scales or fins. That is the aggression of the beani cichlid.
*Nothing is imo meaner than a female Haitian with fry.
Black umbee's are also worth a mention. Other fish such as grammodes and RTM's are usually not outwardly aggressive, but commonly kill tank mates at night.
-Bis