Join Date: Mar 2013
Location: Southern Indiana
The equilibrium concentration of CO2 in a "typical" freshwater aquarium (75 deg. F, ph:7) is generally around 1.5 parts per million. No matter how much you agitate water (in normal atmospheric CO2 concentration of approx. 300 ppm), its CO2 concentration will not exceed this. That's why CO2 injection works so well. It's basically 990,000 ppm.
A dense bunch of actively growing plants can consume most of the CO2 in an aquarium fairly quickly. This is because (unless you add CO2) they are limited by how quickly more CO2 can transfer through the surface of the water.
Plants in nature do not require as high of a concentration of CO2, because they are drawing it from a much larger (practically infinite) "reservoir", and the surface to volume ratio of most natural bodies of water is much, much higher than an aquarium.