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.
You are spot on Highflyer! I get so tired of people calling diffusers "aerators."
