Your fish have a fungus. The fungus is a result of a pathogen. The pathogen is a result of a stressor, like rapid temperature fluctuations. I see this situation quite often, and is most associated with crowded conditions, especially with green sunfish and bullhead catfish. Throw the fish out. As temps drop, the fungus eventually kills your fish, but in warmer water, fish heal before fungus has time to attack. Another of nature's way to keep populations in check.

Teach a man to grow fish...
He can teach to catch fish...