You raise the overflow box water levels to about 2-3" below the surface.
Then seal the dry section of the wet/dry filter with tape or plug vent holes.

What goes in and degasses, also degasses right back in.
The deal is that it still works well because there's rapid exchange in the gas phase for the bacteria to use the O2, which is also degassed and used by bacteria in that chamber, and then the bacteria give off?

CO2, so the net use is less O2 and more CO2.
But if you vent the dry section, you will lose the CO2 quickly.
CO2 is fed directly into the return via 2 reactors and running about 800 gph through both of them.

Regards,
Tom Barr