I currently have 14 tanks running.
Hands down, no question, my male Blood Red Parrots each rule one.
Dangerous, suicidal, Kamikaze, nutcase, knock themselves out against the glass to get you, aggressive. I had to put them in acrylic
--and one of them can open his mouth enough to grab my thumb the BAST@$%!!!!
I also have two Giant male Discus that are complete jerks.
So, that's 4 tanks.
The other 10 tanks are all getting along. At least compared to the 4 mentioned...