I just went through this with a lionshead that was given to me. His fins were rotting off and he was in bad shape. I was told on here to use Melafix and used it as directed for 7 days and the fish is doing 100% better. I also used it on a large 7 inch Tin foil bard i was given. He was on his last leg also.Losing fins,eye was clouded over,he was in bad bad shape.The Melifix had him eating and looking better in 4 days.
I have also used Marasyn on a black moore i took in with fin rot. It did well too,but i like Melafix because it does well,smells good,and is easy to find and reasonably priced.
I would say that this fish was in poor water quality. Probably not cleaned and filtered properly.
Its sad when people think they can just get a tank, fill it with water and drop fish in it,with no more care other than feeding every once in a while.
Good luck!