It might be ick, an infection of the parasite Ichtyopthirius. 8 }( d! D. S5 L' M* J+ q5 N. Y Z8 c0 Y. Y9 }
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. * @' o* A) g2 j $ Q. Y+ `( F7 [8 o( o* j1 F% SIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 1 d* R7 X- b2 k7 t8 {& n2 u# a7 `) o$ q- v! q2 `1 z
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? * j( Q3 s2 W# N5 }+ P9 {/ \. B . R8 Q7 h8 C1 R- D* O) R# Q1 g7 WNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.