fins - To be completely fair to all advisors, I think you should take the time to pose this question back to the retailer and mention to them the warm water refuge concept of winter aeration. Retailer may have a common sense reason/opinion for leaving the diffuser in the deepest water all winter. Please, Come back and give us HIS OPINION or rebuttal.
For reference, also Tell me where you bought your windmill.

As far as I know this warm water winter refuge concept primarily pertains to electric air compressor driven systems. My Rational or Theory::: Most, but not all, electric systems move a lot more water than the windmill units. They also move the water stronger and farther. Amount or percentage of water in the pond moved is a function of 1. the amount or volume air produced, 2. size of the diffuser, 3. size of the pond 4. shape of the pond basin and 5. depth of the diffuser/diffusers.

Movement of lots of water during winter in small ponds such as yours will pump all the water out of the pond bottom, decrease its temperature below 39F at the surface and remix this colder water back to the bottom. The pond becomes completely mixed and few if any areas are unmixed. The colder the temperature of surface air and the longer the system runs continues to lower the overall pond water temperature. I have tested or measured this condition several times.

Any temperature below 39F for the fish is an additional stress that would not have occurred if the aerator was not operated in winter. Even operation of the diffuser in shallow water causes significiant mixing of the pond water under the ice cover. This was tested at Cecil Baird's pond in winter of 2002 when his bass showed signs of red gums or teeth areas.

The more stress applied to your fish the more susceptable they are to disease. Your fish (depending on species present) will be able to tolerate some lowering of their body temperature but to what degree?? This will depend on lots of variables. Some tolerate cold water better than others. Chronic cold water conditions below 39 to 40F are a lot worse on the overall fish's health than an occassional foray into colder water.

Back to my theory... The windmill is a comparatively sporatic and weak water mixer. Thus it may?? not cause undue temperature reduction stress to your fish. I have not tested the water temperature reduction affects produced by the windmill during winter; Maybe I can get that done this winter. Where in OH do you live?

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