for oxegenating water there are 2 main methods, aspirating, and agitating....

Agitators use a small motor to spin a paddle which is in the water. The paddle splashes the water, which adds oxygen and removes carbon dioxide.

(skimmers)Aspirators also use a small motor, which spins a small venturi device. Air is pulled down through the shaft and exits the venturi underwater, causing a draft of bubbles.

Agitators are the best choice for heavy stocking loads because they transfer "more" oxygen than aspirators.

Aspirators are a good choice for smaller stocking densities, and they are much quieter than agitators. And less messy...

this is fact, Bob is corect, but skimmers do the job for us with our low stocking densities....

Oxygen exchange at the water surface is "best", and oxygen exchange underwater is "good"

this is why in aquaculture the utilize foam heads(mazei)(beckett) at the surface and not underwater