It might be ick, an infection of the parasite Ichtyopthirius.3 \! W9 I+ b' d5 U4 n
3 j+ u: _7 v9 I( vYou 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.3 y2 p7 t- Y% f+ N# ]
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It might be hard to keep all the infected fishes, but treatment is necessary to protect uninfected ones. ! Q1 h# K# V! B; Z: f$ Y1 [1 O& M6 P4 }. ^2 M h) n+ k
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?8 S t' v( }; f
, m6 E+ Z. [, r' ^2 f- v/ z: j2 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.