It might be ick, an infection of the parasite Ichtyopthirius.6 g; R- J0 b# z/ K; z
7 o4 Y6 Z+ @+ I: H7 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. B( N! E; y- p: e/ j* k s
" B9 ^9 {6 I. j( M4 s O: aIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 3 T$ R M. v$ P' c " o x9 O" Z' A# R4 `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?7 j6 ^- |' g) q5 J5 N; o
/ y' ?" g9 M# H3 V& b$ g& b8 wNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.