It might be ick, an infection of the parasite Ichtyopthirius. g9 p) i. Y8 d* J5 K) ]8 ?$ x) M8 l+ t+ a
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. $ }! l9 [2 N! M# L 3 F0 q0 ]# C' z8 U4 e M7 u1 TIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.) h- ^$ z! I& \
- ^8 S0 Q1 b- TBy 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? 6 I# p2 O1 v/ E: k& x/ x/ l( ]' m- j( P7 y% t
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.