The soot is invisible until it attaches itself to goldfish then the true color shows, it's really nasty stuff, sometimes it can take 5 years to appear... Just can't heat the pond with coal even with filtration, some soot always gets by and dirties the fish eventually...
![tease :tease:](https://www.worldwidewatergardeners.org/forum/Smileys/smilies_smf/tease.gif)
Does it look like it could be a disease type color change? Fish still acting healthy? I haven't any idea why, but no one else seems to be paying attention. You may want to post the question on a different forum section where it'll get more traffic?