It might be ick, an infection of the parasite Ichtyopthirius. 0 y0 \( U. u) V N% D/ {9 j 4 v+ @/ m) s% B6 B0 `: wYou 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.' x% @- y# }# s/ E3 a; K. I
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.6 Q1 ~" y; l" T5 e
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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? 6 s* L4 r1 G/ E7 v , N) d7 Q! d3 q0 w& n5 K" k) tNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.