An article in the newest issue of KOI USA, by a club member that lives in
Maryland, is very good look at pond deicers, air stones, and pumps. He
argues that you should not use deicers, since they do not create a current
and water low in oxygen, high in carbon dioxide, hydrogen sulfide and
methane will develop in the bottom of the pond. The densest pure fresh
water occurs at 39 degrees F and will fall to the bottom and stay there,
building levels of pollution which can stress, if not kill the fish. A
bubbler, or a pump, located at the bottom of the pond, forcing water to the
surface will take the warmer water and help to melt the ice, but more
importantly, give good gas exchange for the water at the bottom of the pond.
There will be some cooling of the bottom water, by its mixing with the
colder surface water, but the amount is dependent on the size of the pump.
A small pump will not expose large quantities of water to the mixing action.
I think he uses 750 gph pumps. The ground will continue to provide some
heat to the bottom of the pond, keeping it near the 39 degree level.

He also recommended that the salt level in the pond during the winter be no
more than 0.1%, since salt lowers the freezing point, but it also lowers the
temperature difference between temperature of maximum density and freezing
point, so the fish do not have 7 degrees from bottom of pond to surface, but
maybe 4 or 5 degrees.

Around here, in Virginia, most people keep the pumps and filters running all
winter, but we seldom get extended periods of sub-freezing weather. Ice on
the ponds will rarely last more than a week.

Of course as I write this it is 72 degrees outside, record 75 today, but a
front that will bring most of the ponds here some ice within a few days.
The koi and gold fish have a shock coming.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html