It might be ick, an infection of the parasite Ichtyopthirius.. P4 z- G+ A8 a4 w+ @ z) S
% ] f8 t5 L" x7 cYou 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. ) r N6 ~! l* p( f+ N9 w2 A# v- r9 k( U8 I7 ^& _0 Z- N1 N
It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.5 d: z1 m$ G4 ~0 x" 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?6 c2 [# Y( m7 F7 l' ^; }6 C& k
$ [/ A2 f! T6 B- j sNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.