It might be ick, an infection of the parasite Ichtyopthirius.5 H6 Y( O% v7 ^1 g
7 o9 ?' R4 p$ X! {' fYou 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. : P7 A8 H" y' E: M4 J% z' Z ; [& _: ~" z6 eIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 3 P" D) s' O6 t- k 5 o0 n' |4 I$ j6 {. SBy 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?2 O: B3 s8 }$ P' c# N+ ^3 J( o
9 v1 D/ m ?& o2 i) l; ]
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.