Hi again, I have just come across this which I Have not heard of before, It has got me wondering?
Carp pox
Carp pox is a very common problem in ornamental carp (such as Goldfish and Koi), it is caused by a viral infection similar to the human herpes virus. It does not seem to cause the fish much distress and the fish will continue to swim and eat readily, but it is definitely unsightly.
Carp pox shows itself as off white waxy lumps predominantly on a fish’s tail and fins, and is often brought on by stress or dropping water temperatures and often reoccurs during winter only to improve when temperatures rise again. A fishes immunity system will normally fight off this infection within a few weeks but can be helped by gently scraping the lumps off and applying tincture of iodine.