I’ve seen this type of reaction to medication.I just remembered. I’ve had stuff like that happen to these two fish that I can’t remember what kind they were right now. They would develop that same fungusy growth. At the first sign of that growth it meant a big water change was needed. Hours later the growth would be gone and the fish would be fine. Try a big water change. It may be the same thing that affected those fish when the tank was overdue for a water change. When’s the last time you changed his water?
I agree. Look at the emaciation/sunken in belly and 45 degree body positioning. Poor guy.I’ve seen this type of reaction to medication.
I also seen this happen to fish where PH test regent was accidental added to the aquarium.
I agree with the water chang suggestion.
I also see some fin damage that makes me think this could be the culmination of a longer term problem.