Discolouration of Goldfish

Question
Hi. I have a large goldfish and have noticed a discolouration to its skin, almost looks like mildew that you would see on a damp wall and is mainly black. I have looked on the web and can't find anything else that looks like this. Any ideas what it could be? I do have a picture but not sure if I can upload this. All of the other fish in the pond are unaffected and the water quality is ok with no nitrites or ammonia. Thanks

Answer
A photo would help.  You can e-mail one to help at fishpondinfo dot com .  Goldfish change color throughout their lives so my first thought would be that it's a color change.  It might also be injured areas that are healing, for example, if the fish scraped himself.  I look forward to the picture.

Robyn
fishpondinfo.com