It might be ick, an infection of the parasite Ichtyopthirius.: v/ q u9 c& c) t7 L8 o& @3 G
+ J D9 h5 Z& k) S) _* P% KYou 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. . s. e3 a: L. ^8 K3 R# s. V " l. S; g$ Z; R/ \. m2 i0 tIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.2 Q2 e3 X4 K5 N
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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? 4 B" j# [& H9 W4 d* d% j% O3 |' r4 `6 K( Y
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.