It might be ick, an infection of the parasite Ichtyopthirius.6 X9 R; U. n5 J C5 E
( [0 U" G( L3 m; C/ p' ZYou 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.. o( C3 v0 [/ t" b! b$ k" [
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones.( z( Q/ n* G( @8 a# _* c' Y
* g9 b0 o5 J" ^: w0 ^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?1 f; Q# [: u2 [0 M7 u! I4 q
/ N4 G1 @0 b2 i/ lNo, they are cold water fishes, they can survive our winter outdoor as long as water is not frozen and exchange of gases can continue.