It might be ick, an infection of the parasite Ichtyopthirius. 8 t5 J. ~7 u6 B5 e! b* @& a/ m% q: L! `2 F6 I
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. 7 G7 x! H& f3 k2 n1 d ! f: G3 o! W/ D1 M2 |It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.3 x9 m' f* e+ l" }: G
3 ~$ r) n h! T O" lBy 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?7 g M- M7 Q$ ^( q
1 m2 I* P8 h2 H+ P; ~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.