It might be ick, an infection of the parasite Ichtyopthirius. % v1 R) z2 B. w7 S0 ]. Q( O . B" L. w. j# ?7 a7 n- 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. / P' m0 M5 X+ h# b / W' Y+ b+ C% i: j; L' e* I1 WIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. ! e. L; {6 @' _$ L 1 G( z" O6 J% P9 Z. k5 x; IBy 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? ' N% }/ O/ ^- L* X1 S2 R9 L# H2 ]) f
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.