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I think in your case continuing to feed water thru the bio filters at the same speed as before (summer) to maintain water quality is important. Diverting the rest to bypass the chilling waterfall is a good idea. I would not recommend slowing the flow thru the filter to slower than before. In the mid 40's contrary to popular belief you bio filter is still active. The filter handles less at these temps but the koi also produce less so it stays in balance...... If you don't run the BD
with the koi sitting on the bottom, then urine and other pollutants will build up.
My experience says i would rather give a little on temp than water quality. Before i heated my pond, if the mean temp never dipped below 41 degrees my koi got thru winter just fine. Maybe in the future you could plan to give some thought to a protective cover (stilkl venting for gas exchange ) and maybe a small spa heater that could hold a temp overnight......
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