air is roughly 300ppm of co2, co2 constantly bombards the water and there is naturally co2 in there, but plants could benefit from higher levels thats why we inject more into the water. There is a scientific explanation for this, i read it somewhere but could not udnerstand it hehe
anyways if your asking that if you can achieve moderately good levels of co2 using the air we breath then its partly true BUT you cannot go over a certain limit because the co2 in the air and water balances out. (oh ya even if the air is 300 ppm co2 that does not mean that you could achieve 300 ppm with water, its way lower then that.. i forgot the level)
