there are two problems with "under 50" temps, one is their immune
system shuts down, the other is up here in the frozen tundra without
covering and heating the fish must go for 6 months without food and go
thru a long transition to upregulate their immune system. I think it
is hard on koi. My koi are much healthier since I started heating the
water and feeding them most of the winter. Ingrid