It might be ick, an infection of the parasite Ichtyopthirius. 0 [$ k# n2 ?: @5 w5 {8 y3 h* g - m9 h( [# ^# t& wYou 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.! Y: Z( Q+ R N& ]9 K& x# ^
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. : u; U$ T* `3 W ! }$ \$ {5 ?5 D; ?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?- s: W6 _& c" h3 c. z
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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.