It might be ick, an infection of the parasite Ichtyopthirius. 1 V" q8 M& j3 G3 s& ^! x/ h7 a$ X1 i Z9 a' ?3 Y: h* ?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. " u* |( \' @0 G) |5 C ! D5 L! F& j0 s& Q4 W0 q ] X* @It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.- t6 |+ E* ?& }6 `
+ M! X! ?2 p' o r# d: @2 R# l1 P
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?& @9 K0 `$ y: |( M
: g7 V$ d" }6 ~: Z( d6 Z* X* y, O
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.