It might be ick, an infection of the parasite Ichtyopthirius. / J+ _! F2 P! T7 d% u5 y . p/ U4 [7 d$ N2 H5 n$ w. x0 T* JYou 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. 2 x" G: j7 i: G9 T8 n4 V+ @! y4 r! l: B9 F& N# a4 U
It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.4 l9 y0 E3 e8 u- `4 b$ d! j; P
) c0 K% E7 M- ^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 o; @; e6 u d" V0 T& T2 L
5 a( t7 m; u+ j, h2 _+ {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.