It might be ick, an infection of the parasite Ichtyopthirius. " \' R1 c+ M! Q. ]3 M! z) a( E+ v: Z$ a% N* R( c6 K- A! Q
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! h* o% ^3 M3 f) g8 y
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. 2 j8 p9 d7 \" c7 z7 a. w) r( s& R ; d4 x' L; J$ b) m' F- k7 q! G7 `5 wBy 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?3 K7 f9 c* I; X S
, x+ v$ {( n- D% ^+ kNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.