I've attached a pic of a poorly fish, he's been lying on his side flat on the bottom for a couple of days and I thought he was a goner, but today I found him swimming about and feeding again.
There's a obvious sore patch on his side (which used to be all white).
The large pond has been established for about 15 years, is crystal clear and I've had no issues - with two curious exceptions below:
Last summer I noticed one of the larger koi spending a lot of time sitting quietly on the bottom and it had a stiff swimming action - he seems otherwise healthy and happy.
I also have a grass carp (I think!) with dark bronze scales and I noticed what I thought were white fluff balls on it's head an body but they weren't - they were just clear white scales that had appeared, most odd but, again, he seems otherwise healthy and happy.
All other fish have no issues that I can see, I've had them all a long time now.