I have 3 fantail goldfish. The fish most recently added about 2 months ago has recently had a black mark showing up on one of it's side fins (coming out of the side of the fish). I had problems with this before with other goldfish and actually lost 2 fish to it. I thought it was a parasite so I had treated her with a parasite treatment. The fish don't seem stressed but the black marking is not going away. The mark isn't getting any larger but I'm concerned because I don't want it to get worse, or infect my other fish. Any ideas?

There is nothing wrong with your fish hon. Its very common for a fish to change color when their environment changes. I once got a gold fish who was complegely gold color, and then once I got him home, he slowly changed to white. And several of my breeding friends had fish who would be one color, and then change color for no reason at all.

If you are truly concerned about it, take him to a vet. Even if you can't find a fish husbandry vet, all vets have basic knowledge.

Good luck hon. Letme know what's going on and if anything changes.

Just a little story to add to Jennies answer, I have 2 goldfish and one of them was almost pure white when I bought him with a little orange spot right between his eyes.
A year later he is almost completely gold with just a little bit of white underneath his belly.
Fish change as they get older and/or their environment changes.
Back when we had Discus they used to change color all the time, so much so we would get them all mixed up!