Mine was a 12cm jungle perch. Little bastard killed everything bigger or smaller than it. If it tried to dig a hole and hide, he'd go and dig after it. Good thing he didn't have teeth, so he couldn't hurt plecos, but he did put countless rainbows, an archer fish bigger than it and a small Hyrtl's catfish to bed amongst other things.
When I finally decided to try to put a tarpon in the tank, he beat that up too, so I moved him into a bucket for a few hours. Of course, the stupid jungle perch jumped out, and wound up dying.
Now, I have two indo-pacific tarpon in that tank that of all things, eat flake food. It's nice to have mellow fish in a tank again...these guys don't bother anything, even if it's small enough to eat.