It might be ick, an infection of the parasite Ichtyopthirius. . K5 ^2 V6 m0 n8 u* X" A. y) C6 {; `% ^4 q. |* o
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.6 A" s. ?' S) y5 _+ q. l8 M' A
! ^' s. x6 w8 v. lIt might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 9 M6 W( A7 A/ i! U1 G/ z8 c" N 5 v* b$ S/ x3 A/ X& J" ?/ |1 K ABy 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?' v; Y' o8 V+ F5 ?3 ^" y, T. R# f
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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.