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I agree with Mike, this seems to be a virus desease due to winter conditions. If you want to treet that koļ, you should give injections of antibiotics to prevent bacterials complications and to help the skin cicatrisation. Have a look an koivet.com and read the "Creamsicks head Ulcers" and the treatment.
I don't know if you should kill that fish to prevent the contamination of the others, because it is difficult to know if others or maybe all others koļ are already infected..., but for sure you should isolate that fish waiting what will happen to the others.
If you don't have any other problems with the others, I would do like Mike and kill that fish to prevent the contamination that you will keep for years.
Good luck
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