It might be ick, an infection of the parasite Ichtyopthirius. . B1 N) j% x+ e( B6 h5 F; W: n* x p4 B3 G3 _7 t
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.. ~0 z: I8 _6 L+ y. ]0 i& }7 S0 T% R- U# W
$ s) p! x: ^6 ^0 \It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.' S& u+ B5 c }" B4 [/ \6 A" I8 o& Z
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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? . [; q3 f1 g: O j. O4 w% h. O( G% t. l" o0 I4 H' d
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.