It might be ick, an infection of the parasite Ichtyopthirius.( G0 J( X! F# e% G' n6 e8 t8 ?
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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 v2 ~; ?& c; H0 l7 K9 y7 r U8 B' g( b" n4 \4 g
It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. u0 s9 t) M' ?7 ^4 s8 }8 _' Y7 X- {7 y4 x$ U
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?% R2 T W1 c+ ~- t1 y7 [
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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.