Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Water holds just so much of each of the gases.
Where water contacts air the gases can enter or leave the water.
If something in the water is removing certain gases (plants remove CO2 from the water) then every time the water contacts the air more CO2 will enter the water.
If something in the water is adding certain gases (plants add O2 to the water) then every time the water contacts the air more O2 will leave the water.
If the water is not moving at all, then the only part of the water that can exchange gases is the part in contact with the air. Gases do not move through the water very well, so for example, and CO2 the water gains from the air stays within a fraction of an inch of the water surface.
If the water is moving with a gentle ripple most aquarium keepers find that is a good amount of surface movement. Enough to keep away any surface scum, enough to ensure good gas exchange.
I generally set up my tanks with about 10 times the tank volume per hour of water movement. Some of this is from filters, and the outlets are near the surface. Some of the water movement is from power heads that are lower in the tank, and there is not a lot of surface movement from these.
Water circulation in the tank is also moving fertilizers around so that the plants have a constant source of CO2, fertilizers, minerals.
Too much water movement can stress the plants. The leaves of the softer plants should be gently moving. Stem plants should be gently waving, but not much.