It might be ick, an infection of the parasite Ichtyopthirius. 6 A" o' f- ?1 Y . Z1 p6 T7 S+ x; PYou 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./ [% X o9 n' S: x. q6 o
% @8 u# N' z$ rIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. h9 Z- V: Z) h& b: t % S& O- L2 T( tBy 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?( \' c: A! n E! h4 A# ~+ ^0 Z( m
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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.