Lew -- the supercooling may occur just because of the water circulation. Water is heaviest at 39 F, and that water is on the bottom of a pond during the winter. At the surface, of course, it is 32 F where it freezes. The supercooling thought is that too much circulation brings up too much warm water to the surface where it cools. As a result, bottom water get colder than it should, which may be hard on some fishes.
I'm not 100% how much aeration/circulation it takes for this to occur. Others on this forum know more about aeration systems than me, and they may chime in. However, if it matters to you, we are putting a new windmill aerator in this year on that 17-acre pond that I mentioned to you earlier. To be safe, we plan to put the diffuser in only 6-8 feet of water, rather than in the 26 foot deep water down near the dam. We've got one of those Vertex diffusers from Ted Lea (he helps on this forum), which has very small pores. I'll be interested to see how it performs next winter.
