Air is about 0.05% CO2, so to get one milligram of CO2 in the water you need to add 1/.0005 mg of air, or 2 kg of air to the water! If all of that CO2 dissolved, which it probably would, you would have in one liter of water, 1 ppm of CO2. In a typical 100 liter tank you would then need to release 4000 kg of air to reach 20 ppm of CO2! Other than that, it is a great idea. (This is a crude calculation, so don't go shopping for an air pump based on these numbers.)

Edit: I think I'm off by three decimal places! It is 2 grams of air to yield one miligram of CO2, so it takes only 4 kg of air to reach 20 ppm of CO2 in a 100 liter tank. That is still an awful lot of air.