It might be ick, an infection of the parasite Ichtyopthirius. ) N8 f# J# @7 F* {& N M3 ^$ U& J( }* O: F
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.0 \8 F- h4 N7 t) l% m
9 B9 F5 p/ Z1 t0 U$ f( _3 wIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. ' l8 Q' L' |0 K8 ^4 d& | & V* F) B% ~( M9 Z4 cBy 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? - u4 e) X( J6 H, g( ?7 t * u0 q& ]; {0 @2 ]/ w @! e8 \: iNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.