You can call me a knucklehead if you want but as an engineer I believe something after I see two things. One is the physical evidence, second is the numbers that verify what I think I’ve seen. You all have conveyed enough personal experience that indicates air diffusers don’t add much DO to a pond. This idea is new to me. I have no reason to doubt the physical evidence that you have kindly conveyed but forgive me I still need some numbers. Let’s take a hypothetical situation where we have a 1 acre pond, average 6 feet deep, deepest point 10 feet with a bottom diffuser, 2cfm with average bubble size of 0.5mm. I think we’d all agree that aerators work to whatever degree they might by increasing the surface area of the water exposed to air. So my thought is that it might be revealing to look at how much surface area is exposed to the air pumped through the bottom diffuser in the hypothetical pond. It’s easy to do some math and figure out how many air bubbles are created by 2cfm air and what their surface area is. My scuba diving experience tells me that small air bubbles travel upward in water at about 1 foot per second. This allows me to figure how many air bubbles are rising through the water at any given time. The surface area of these bubbles turns out to be about 1240 sq. ft. This is equivalent to the surface area of a square pond that measures 35 x 35 feet. This is approximately 3% of the original 1 acre pond area. I reason therefore that the 2cfm diffuser is equivalent to adding 3% more surface area to the 1 acre pond but it of course doesn’t change the pond volume. It turns out you can accomplish the same thing by making the theoretical pond 2 inches shallower and adding 3% more area. Even my gut tells me that this is not going to help add significant amounts of DO. But what about the numerous sources I’ve found that show how to calculate the amount of oxygen a diffuser system can add to water and the fact that tank farmers use this to size their aerators? This seems to contradict the idea that diffusers don’t increase DO. Well, the key is size/volume. If our pond were much smaller the diffuser may add significant DO. Let’s say the pond were only 35 by 35 feet to begin with. Now this same 2cfm diffuser effectively doubles the surface area of water exposed to air. In this case the diffuser might add significant DO to the water. We could accomplish the same thing by making the pond twice as big but half as deep. I think we’d all agree that this would improve the ability of the pond to pick up oxygen. This thought process seems to explain not only your observations but also the claims that some ponds do get a DO benefit from diffusers. The key is surface area to pond volume. We’ve all had an aquarium where fish died when the air pump was shut down. My rationale even explains what many of you have described as the main benefit of a diffuser in a large pond. That is that a bottom diffuser needs to move a lot of water to be effective. The numbers prove that the diffuser cannot put much oxygen into the pond directly but I would imagine that giving more of the water time at the surface allows it to pick up air naturally (some day I'll tackle the numbers on that). My conclusion is that surface area is what gives you more DO and that a diffuser cannot possibly increase the surface area enough to make a difference. But it can bring up to the surface water that normally would not be exposed to air. So I’m with you now, we need to move more water, or make our ponds lots bigger. Hmm, which would I rather do?

Gotta get back to fishin!