Sick Angle

Can anyone HELP!
I have a sick angle he is about palm size, is not eating, keeps lying flat on his side and his fins look alittle damaged but is hard to see as if he is not lying flat he is sitting on the bottem.
My tank is 4ft, I have isolated him in the tank and used stress coat.
I have not had any problems with fish getting sick just been eaten.
Recently I purchased two convict fish and about a week after introdusing them to the tank I notice the angle. I have another angle who is ok.

Although I have never raised convict fish, I understand they are pretty aggressive. I'm a big advocate of keeping angelfish only with other angelfish, because they tend to get diseases from other fish, they're susceptible to fin nippers, and they like fairly warm (80 to 82 degrees) water. Cory cats do well with angelfish, as do a few other species. Angelfish, though, are much better off with other angelfish.

If you take the convicts out of the aquarium, the angelfish may come around. A 20 percent water change wouldn't hurt either. Any change in water temperature should be done gradually over a day or so. Good luck, and let us know how the angelfish is doing.