I think it's tank size. I see fish in my tank very often will flare up at each other for 10-15 seconds, and then turn around and go to separate ends of the tank. I think they have room to get away from each other, so they will.
I have a Melanurus and a Zonatus in my tank, they don't like each other at all. I don't see how I could keep both those fish in a smaller tank.