I am in Milwaukee, zone 5 and I cover my pond with plain dispo plastic altho I too
make a lean too above the pond.
http://puregold.aquaria.net/mypond/winters/winter.htm
this last year I put in a 500 watt heater for 1600 gallons and it kept the water 50o
or better for all but one month. meaning I could feed the fish lightly and their
immune system was down only about a month. I have a big 12" airstone blowing air
into the pond all winter. the air pump is in my garage.
Ingrid

"RichToyBox" wrote:

My pond is covered and heated during the winter. The cover is a lean-to
with 3 layers of poly sheeting and one layer of the pool solar cover. I do
not leave a venting area around the perimeter, but do enter the enclosure 2
times a day for feeding the fish. I do not think it would be necessary to
provide any ventilation if the cover is mounted above the pond. If it is
layed on the surface of the water, then I would leave at least an inch all
the way around and install airstones. The water will not be able to have
gas exchange under the cover, and will get fouled.



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