It might be ick, an infection of the parasite Ichtyopthirius.. I1 ] ]9 m; C4 R$ h
9 s( F- Q& e) P5 U5 KYou can increase the water temp up to ~28C and give them drugs (basically Cu2+ ions), plus large quantity of water change. Google for further details.$ p! H D* L+ N9 I0 e- v
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.; C# i7 s1 g* M5 c+ u p: C
4 h( R; \! T7 P6 Z8 ]By the way, is it necessary to have >20C for koi and goldfish?
Quoted: By the way, is it necessary to have >20C for koi and goldfish?9 X. ?; V; T+ W. k6 B2 Z
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No, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.