I think this is the first time since I got wireless thermometers that we've
gotten this cold (SW Washington State) without snow cover for insulation.
The lily pond that just has a 100 watt heater + bubbler is holding fairly
well close to 40F at the bottom.

The koi ponds, otoh, with slow moving water and a 1,000 watt heater is
slowly losing heat. The heater on a calm near freezing day will actually
drive the temp up to 40F before I turn it off to save power. Our current
temps are in the teens, much less w/wind chill (thus wind blowing any heat
away near the surface) and the temp is 34.7F for the koi! Brrr! ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us