Yea, discus can be mean to each other. I bet he'll be fine once the others settle in and you put him back. The thing is, discus are tough to mess with once their all established. The best way to do discus is to put them all together at the same time, not with established guys alread in your tank, and either leave them alone or wait till two pair off and sell the rest. Either way though, in time he will get used to the others when you put him back. and I dont think hes chasing his reflection, cause they cant see their reflection. We only see a reflection when we are looking through two panes of glass, which creates a mirror image when theres water involved. They can see right through.