Obviously, cichlids will probably be the majority, but I'm curious since every fish has their own personality. Personally, the most aggressive fish I've kept was a female dwarf gourami. Tried her in all the aquariums I had, and she'd attack every fish. One day, I put her in a tank with a paradise fish, and he kept her at bay for awhile, but she eventually started relentlessly attacking & shredding his fins. She eventually got to the point where she was so stressed, it left her open to disease. It was very sad because she was a beautiful, pure silver color. Anyway, what are the most aggressive fish you guys have kept, and did you manage to calm the aggressor?