It might be ick, an infection of the parasite Ichtyopthirius.! s* {5 T2 T3 l7 l& Q& |& `; @+ j
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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.; W* L( W) p) i; k
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 8 P: r! E1 a/ y9 T5 \) ^: O " [- k& }" y6 e2 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?, j7 w2 l$ x& d' z! x) l
( ^! C0 e1 O7 u$ ^. rNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.