It might be ick, an infection of the parasite Ichtyopthirius. " j6 y& ?* P/ U; Q5 U; r% Y5 X3 J2 \* N4 ^
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.0 e4 @% e+ `# W) v
0 O' G; S' y3 L2 b4 `) U: k7 cIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. ( Y6 p0 B% A& s. n# j' O' f3 C% ]6 A
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?8 p M9 h) X) N- L4 N# G/ [7 Q
$ H3 a: ?3 V2 V8 qNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.