Well I have the most comments on this video about wintering koi. Many of my followers now winter their ponds all winter as I do and never get sick fish. Koi are cold-water fish and the ice cold running water prevents parasites' that make Koi and goldfish sick.
In the UK where they have an already mild winter compared to most others, they heat their koi ponds to summer time temperatures, they like tropical countries all have to fight koi parasites that make cold water fish like Koi or goldfish sick.
I have been watching lots of koi keeper in the UK and they all seem to have sick koi every spring as the parasites go wild in the warm water all year. I have never had a sick or dead koi in all my 30 years of keeping koi in ice cold running water each winter in Canada.
Comment below if you winter you koi or goldfish in cold water and if you ever get sick koi or goldfish. If your water is ice cold and running all winter you should not have any problems.
I know believe its better to keep pond water running all winter than to actually heat. I am changing the way people thought about ice cold running water for healthy koi all year!