Went to a local pet store, and saw a real badly treated goldfish. The goldfish is actually huge, around 6-8 inches. Its missing some scales and the fins aren't look as healthy as they should be. Is there anything I can do to make it feel better?
Melafix would help, it can do no harm to the fish. Also, just keeping it in a suitable sized tank and cleaning it regularly will probably have this fish looking better in days! Good luck and keep updating.
Check out some websites on good care for goldfish, there are lots devoted to them! They thrive on a varied diet with things like high quality flakes, peas (with the skins removed), frozen foods like bloodworms (meat foods in moderation), etc. Good food, plenty of space, and clean water will go further to help your fish than anything else. What size tank do you have it in? It would prefer at least 20 gallons.
I think for fancy goldfish (not comets) it is okay to use a 30 as long as it's not a restrictive shape such as a hexagon. A 55 of course would be even better.