Bala Shark (Silver Shark) Wound..

[Diesel0407]

I took on a 5" Bala Shark (Silver Shark) about 2/3 months ago due to him being attacked by Tin Foil barbs in his previous tank, he had a little open wound near his rear fin his starting to lose scales and the wound has gotten bigger after it started getting better, a local aquarium suggested I treat the wound with Melafix but it does not look like it has touched the wound. 
He is currently in a tank with 8 goldfish until he heals to go back with the other Bala Sharks in a bigger tank. He was with some tiger barbs until we noticed that they started fin nipping, so i removed them into a different tank as I didn't know if they started feeding or nipping at his wound. He swims fine but I am concerned about it getting bigger, I am planning on taking him down to a local fish shop for second opinion. The goldfish leave him alone and tend to mind they're own business so the only things I can think of is maybe the tiger barbs had irritated it by pecking, his catching it on some rockery or maybe his bumped into the heater and possibly burnt himself? I am debating to stick him in a tank by himself to see if it makes any difference?.. 

 

[Ninjouzata]

How big is the goldfish tank? Water parameters?
Quarantining him would probably be a good idea, though.

 

[Unseen]

I would suggest a different tank by himself. I don't know much about gold fish but have read that they can host certain diseases and is the water temperature a little less for them too?