Bottom diffusers lift water allowing it to be exposed to air. During that proces that water is exposed to less pressure which allows it to off gas the bad stuff. While are the surface it is also allowed to pickup some O2.
Meanwhile, the water that was at the surface when this started will be circulated towards the bottom introducing some more O2 to the lower levels which is beneficial.
Surface aerators add far more O2 (due to the added agitation and air exposure) to the surface water and starts a circulation process that will also move saturated water to lower levels.So in short:
bottom diffusers = water movement to surface
Surface aerators = O2 injections to the top water and circulation.
FWIW Fish farmers use far more surface aerators.
Hopes this helps.
BTW:
For Georgetopia, I noticed and immediate increase in the O2 saturations at the surface first and all levels with in two hours using a surface aerator. In fact my numbers were so impressive, I checked my calibration three times and sampled the water in the pond three times over the course of a day before posting my results.
I highlighted part of the quote because it's partly true. In ponds that are deep enough to develope a thermocline, surface agitators will just move water around (with the increased O2 levels) above the thermocline. Bottom diffuser aerations systems homogenize the whole water column, from surface to bottom, distributing the O2 laden water thru the whole water column. It also distributes the higher upper surface water temps thru the whole water column too, which is NOT what you want for trout. Not because it's bringing warm air down deep, it's mixing the warm upper water with the cooler bottom water. The temp won't be as warm if the system wasn't running at all, but at least in my experinece, it warmed up the water too much for the trout to live in. That's why I'm experimenting with hypolimnetic aeration this year in my pond.
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