The surprising thing for me, but observed by many , is that fish rarely fight in front of you. To catch them it is often recommended to set up a camera. They are almost like kids, waiting for the teacher to turn his back, to start misbehaving.I did notice occasionally that the bigger one did chase the smaller one for a second or two each time. But for the most part it was a rare behavior I've observed only a few times and they seemed to pretty happily cohabitate otherwise
As you did not see any wound, I will guess, that it was bullied enough to cause high stress, even though it did not hurt it physically.