I am in zone 5, Milwaukee north of you and to keep the pond clear of ice all winter
is cover it with a plastic lean to and run an airstone and/or a pump. The freezing
does not actually get that deep. That is how deep it has been recorded, probably
during the "little ice age" and no snow cover or out in the middle of a farmers
field. this is the depth used for burying people so their coffins dont get heaved
out of the ground by frost.
all summer long my pond heats the surrounding soil. the 1.5 feet sticks up out of
the ground is insulated.
in any case, 500 watts did 1600 gallons kept water at or above 50o all but one month
this last winter. Ingrid

(MC) wrote:
I believe the frostline in Chicago is 36" meaning that the ground
freezes up to 36 below ground. In zone 7 I could see the ground
staying 55 degrees, but not here.


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