It might be ick, an infection of the parasite Ichtyopthirius. 2 S% x2 H! \7 O* l! V3 ~) V3 C+ ]4 w$ i- e2 A
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. ' l6 ?1 K6 K# O - W# X: J% [: s! h$ {It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.. m( W5 o6 n) F
X3 W1 f# V' T& ]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?- ^$ Y' B2 Y' {1 Y# |6 D8 q
, \( t1 d9 H+ D( A6 L. E- u: @# yNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.