Distribution constant and extraction efficiency. You are assuming you can get all the CO2 out of the air and into your water - not true. The ratio of CO2(air) to CO2(dissolved) is constant for a given temperature and amount of CO2 in the air. You have to change something (amount of CO2 in the air for example) to change the amount of CO2 in the water. By bubbling in pure CO2 you drive the system out of equilibrium and temporarily add more CO2 to the water.OK, just did some quick checking on the levels of CO2 in the atmosphere. Apparently they are somewhere above 300ppm or higher depending on location, weather, etc.. Now I don't use CO2 injection on my tanks, so I don't claim to know much about that particular subject. However, it would seem to me,... and obviously I must be missing something,... that we could eliminate all that fancy CO2 injection stuff, simply run airstones into our tanks and get tons of free CO2 as a result. What am I missing here?? If you typically shoot for a few ppm's of CO2 in a tank for good plant growth, and the air outside the tank is loaded with the stuff, relatively speaking, why spend money to BUY it??? Clearly I don't understand the process.