It might be ick, an infection of the parasite Ichtyopthirius. 7 r" R7 e% @( F+ i1 Q : t9 C' J7 C9 i/ R+ S% Y6 M r$ ZYou 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.9 q) B c) f. o3 O5 Q" a
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. + _* f$ L2 b. U( t8 e. m O Q- w' W! I
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?/ b3 l6 r9 r3 [* ]/ g. E
' A$ y/ Z/ f+ G" z. i+ Q5 `8 fNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.