It might be ick, an infection of the parasite Ichtyopthirius. % n( q8 W" {) Z/ b8 L. W % }; G3 [- n% u- j) h- 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. ) T7 d2 F5 R/ ]7 |" D& c! }5 Q" M7 G! z
It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.0 y5 l- C: F4 s# w7 j
& D1 E- B! d6 B3 K1 zBy 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? / o# R& s3 I+ e! ]# `) y' T5 M \) |! m: z% ?+ y- o3 J$ M
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.