It might be ick, an infection of the parasite Ichtyopthirius.5 A( B( O3 t3 M% O4 {
% Q3 X2 h0 q; d2 X
You 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. ( \& V7 b" b& v# ` Q! G4 T* j- y* K; y2 o# v! U
It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.! R) e4 k9 [- C7 H; F4 H
6 L+ q6 I8 F: x2 ]
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? ) P$ u) n& P/ `3 V) C) ?8 @% p : t/ X" {$ I, z% m; }. m) gNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.