Make the edges of your pond not vertical 180 to say 120 degrees. This
way Ice can rise up and down instead of side ways. Very important with
concrete ponds.
I use a horse water heater so the issue is mute here in zone five.
Comes on at 35 degrees and is really meant for horse water buckets but
it does the job and my Koi winter over with no problems.

Bill

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S Jersey USA Zone 5 Shade
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