It might be ick, an infection of the parasite Ichtyopthirius. e5 N2 i8 q/ ~& J h2 y* p. q8 x) ]' H. d
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.8 g% V4 {4 G6 \7 i8 l7 s4 O$ s
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. $ z/ e( v% T8 e7 I( F5 N: C) h2 i7 v/ P/ t% \! [$ r0 p9 h0 Q
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? " J2 a) i" @! h9 d( U! r7 F) h' u1 {6 D8 C' `6 o7 E
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.