It might be ick, an infection of the parasite Ichtyopthirius. / @! f5 f% x+ r1 \8 s l: }9 {& B. O3 i% m( ]7 C: ~8 `5 A {
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 [- v. C0 m1 a; t
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.; A1 y6 C0 D' {+ \( R7 O! g
6 p7 a0 k. } o8 O2 B zBy 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? 5 @) a1 ^+ l* R8 v7 ?$ B, q. v! z) Z% z) e a% i9 F
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.