No idea. I've been told by various people that it won't freeze if I keep the bubbles going, but it does seem to me that there should be a limit to the temperatures at which this would work. The pond is sunk four feet in the ground, which is deep enough that most of it will never freeze here simply because of the ground warmth (frost line is 18" in the outer areas, probably only 12" where I'm at, and then, only in a particularly cold year--many years we only get a couple inches of frozen ground). I've placed the airstones in one corner, about three feet down, judging from the amount of airline that went in, hoping that it wouldn't stir up the sluggish fish or their warm spots. I don't know if the effect of bringing up ground-warmed water plays a role in keeping a hole in the ice.
We've seen temperatures in the low 20s since I put it in, but nothing sustained through the day under 25F or so. So no good data yet. I keep going out every couple days to see if the hole is still there in the early morning.
