But to raise the Oxygen and drop the CO2 out of your tank, you can promote gas exchange with surface agitation and good brisk circulation. This will in turn help keep your PH higher. Sand will also act as a buffer. I have always kept at least one powerhead pointed where it will move the surface water and keep my PH stable. You also want to make sure your house hasnt been closed up for a long time. Low amounts of oxygen in your house will effect the PH in your aquarium.