It might be ick, an infection of the parasite Ichtyopthirius. 2 Q9 Y# x. l s4 y/ I 4 d" @. W" W* H5 `4 w( @8 sYou 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. 6 s% @: x9 R) W# l+ _0 X( N $ t8 o* Z+ G5 D2 \3 R/ o5 p3 S( iIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 5 w5 a+ r! y0 a. f+ o! _2 M) O7 [! Y& h G' X2 U
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? / n( u+ W+ s5 T& {8 _; q4 p5 N! z t/ K
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.