[sumpthinfishy]

Okay. I have a goldfish that was in horrible conditions. After she was here and in a good tank with good water conditions, she developed a black spot on her head, on the edges of her tail and dorsal fin. It grew over a period of several days. After a couple of weeks, it gradually started disappearing. The edges of her tail are now white. I don't know if they would have been white anyway. I'm guessing, yes.

Anyway, the research I found is that it can be caused from ammonia burn. It only appears AFTER the conditions are improved. It looked like ink smudged into her skin. She has no more black and is perfectly fine. Hopefully that is the worst case for your fish.

I think there are other things that can cause a black or dark color on goldfish from illness, but am not familiar with it.

But I've seen goldfish change colors as well.

However, in the case of my goldy, it was not there one day, there the next and spreading.

Check google. I found a few really good goldfish sites with info about disease. May want to do a search for ammonia burn in goldfish and see if that brings up anything.

Hope all goes well.