Experience at my 1/4 acre pond has proven that the 3 diffusers, set about 3 feet off the bottom, turn over the entire water column (no thermocline retained). One head in 10 foot of water (7' from the surface), another in 7 foot, and the 3rd in 6 foot...equally spaced on the dam half of the pond. I have taken temp readings (top to bottom) during initial start-ups that indicated that the system turns the entire water column over in a matter of hours. Within hours, the entire water column is the same temp top to bottom (within a couple degrees). The idea of a retained thermocline, with an adequate air diffused aeration system, must only be possible in larger bodies of water with substantial depth. I have yet to experiment with any heads shallower that describe above.
My concern regarding running the air less than necessary (whatever that is) is that it would not be bringing bad water up enough to make it good water and keep it that way (This is where, I assume, the "Turnover per Day" rule of thumb comes from). Hence, periodically bringing worse water up and mixing it with good water...making good water less good. I'd rather have good water at the top and bad water at the bottom compared to mediocre water throughout (and cyclic as well). I am not saying that I think this is what's going on with Tracy's pond...just making conversation and would love to hear others opinions on this topic.
I determine my air run-times during the worst of the summer by comparing the ambient air temps at night to the water temps of the pond. I will run my air at night as long as the air temps are below the pond's water temp near the surface (about 12-18" down). Example: If the 12-18" deep pond water temp is 85 degrees F...I don't turn the air on at night until the air temps are well below that (5 degrees or so). Air temps less than water temps means you will be cooling the pond by exposing the warmer water at the cooler air at the air to water interface. Overnight, my pond temps equalize top to bottom due to the turnovers and I will turn the air off in the morning as soon as daytime air temps exceed the water temp. This is where the '"art" comes in along with some other forms of magic...I, obviously, do not change my run-times daily. So, I have to watch/predict the highs and lows for the month and set the system accordingly with the timer. The depth at which I measure the water temp could be different for every pond too. It all depends on how the temp gradient is from top to bottom by the end of the day while the air is off. I try to find a depth that represents an average temp.
