It might be ick, an infection of the parasite Ichtyopthirius.+ D. B5 S' P5 n4 @* {& s! Q
7 l# \ g/ ~6 [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.' Q7 m% a" J9 \+ t
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.* A9 b# m' a( V+ h+ h
$ g) z: N3 Y$ ^9 hBy 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? $ O" d( L/ k R9 p8 m) ` ) z$ B+ f9 d' D7 JNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.