It might be ick, an infection of the parasite Ichtyopthirius. 8 B6 E h- p0 N# P5 D% B 2 s! S6 i+ ^3 ~* a4 N% `: TYou 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.( J3 O$ J$ H! ~/ ]
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.0 c! {9 I X9 U. T9 ^9 N: l
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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?& Q' G; b N% m. U0 p/ ~. y% w
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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.