high quality food with pristine water conditions are more than enough to give you a beautiful healthy fish. Tea tree oil should be safe as many herbal medications that claim to be reef safe contain it, but im not sure whether it will make fish healthier, i doubt it.
I used it in fresh water some years ago, though never on a reef.
My experience with Melafix was that it gave a distinct odor to the tank, and that it was somewhat foamy. I don't think it'd get along well with skimmers . . .