It might be ick, an infection of the parasite Ichtyopthirius. 5 S6 h7 ?( t0 N2 ^ 4 V5 b/ j! c* T- EYou 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. $ L3 t: j' B# ?$ h! u9 x + W4 a9 I1 C+ K+ f j5 `- `) qIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 1 R- B7 [/ d* N 6 _6 ?0 ? F6 v8 JBy 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?1 E9 x( V2 t j: t+ M k. y$ a
6 Z% V. V0 D5 _2 SNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.