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Natural CO2

We know that agitation or aeration of the aquarium water
drives out the dissolved C02 in the case a CO2 fed tank.

The converse is true in an aquarium *without artificially added
CO2*. During daylight (lit) hours the plants absorb CO2
dissolved in water for photosynthesis. CO2 levels in water will
drop to levels lower than the equilibrium with air. CO2 will
then diffuse from the atmosphere into the water..

At night the O2 levels will go down as the plants use up
Oxygen and release CO2. CO2 will be higher in the water than
air, diffusion will occur from water to air.

         Daytime                Night
CO2  Air  => Water         Air  <=  Water
O2    Air  <= Water         Air  =>  Water

We can speed up the process and make more CO2 available
to the plants during the day and more O2 for the fish at night by
either adding an aerator or a spray bar to the outlet of the filter.
I am in favor of the spray bar.