One of my oscars has pop-eye in one eye. I am treating with myxazin. He has had it for 2 days and is eating and acting normally

Ammonia 0, Nitrite 0, Nitrate about 25 (last water change tuesday-3 days ago)

The oscar with pop-eye is 4 inches long and he is in with some bigger oscars. The largest is about 9inches long including tail. The oscars are not really aggressive but there is some occasional nudging, shimmering and mouthing(but no mouth grabbing)

I am guessing that the pop-eye is caused from some fighting. What do anyone else think

Popeye is a disease. Poor water conditions can cause it.

Someone once told me that fish can get a swollen eye from fighting where one fish attacks another and hits them in the eye

My nitrates are always around the 25 mark, my nitrate tester has colours for 0,12.5,25,50 and 100. The colour is usually around 25 and never meets the 50 one. The amount depends on how long it was since I did a water change. I use a tetratest liquid tester and I follow the instructions exactly. I do a 50% water change every week without fail. If its not fighting that caused the pop-eye then where have I gone wrong with water quality given that ammonia and nitrite are zero and nitrate level is ok?