It might be ick, an infection of the parasite Ichtyopthirius.9 Q9 Y/ u6 J6 L3 f# K
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You 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. 7 Z4 l' ~2 H% p3 k+ Z1 a ; ]3 e; T: T4 {% E' p1 S' ]( bIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.+ h* { w' G& q7 S& c8 `/ d2 k
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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?9 A4 J2 F& j9 g8 a1 k
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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.