LOL parasites, fungal, hex, viral, melifix, metro. You got all kind of advises or just speculation. I don't think anyone knows what is causing it.
I'll offer my 2 cent advise. If all other fish are doing fine, do nothing, observe and keep up with regular water changes and hope the fish will recover itself. Without diagnosing what is causing it and applying medication indiscriminately to the entire tank with healthy tankmates may do more harm than good. If the fish gets worse, isolate him in a hospital tank and try some broad spectrum anti fungal anti parasite medication and hope it works. From time to time, fish can develop congenital diseases that cannot be explained and not necessarily contagious.
I'll offer my 2 cent advise. If all other fish are doing fine, do nothing, observe and keep up with regular water changes and hope the fish will recover itself. Without diagnosing what is causing it and applying medication indiscriminately to the entire tank with healthy tankmates may do more harm than good. If the fish gets worse, isolate him in a hospital tank and try some broad spectrum anti fungal anti parasite medication and hope it works. From time to time, fish can develop congenital diseases that cannot be explained and not necessarily contagious.


