Orange On Orange Violence?

[SLPReefer]

I have had two 1.5 inch percs in my tank for about 4 months. They have always been friends. Yesterday I saw the bigger one chasing the other one and being very aggressive with it. It would push it into a corner and make it stay there. I though that they were just establishing dominance or something so I let them be. I came back later and they weren't chasing anymore, but the one that was getting chased was now resting on some rocks and breathing really hard. He was still swimming a little though. He proceded to get more lethargic so I looked up some stuff about diseases and thought that he may have some sort of gill parasite. I did a 10 min bath with formalin/malachite green water, and then a 4 min buffered freshwater bath. He seemed to be breathing a little better after that, but by the end of the night he was dead. What happened in my tank?

[David Grigor]

It is pretty tough to speculate with the information given. Likely will never know if the chasing caused to run into something and cause damage, the other clown itself caused damage or if it were just coincedence and was ill. Usually at the point of breathing really hard, it's proably too late at that point and your bath did little to save it.

[SLPReefer]

Is 1.5 inches still small enough to be able to find a mate for the other one? And is this common to have fish get along perfectly for a few months and then get aggressive? I never had anything like that with freshwater.