HD and Dark Cobra are right. The density of water does not allow CO2 to be absorbed very well by plants. You need 10x the amount (concentration) of CO2 available in the air going into the water column. So using plain atmospheric air is not a viable option. If that was the case, we'd all do it.
You can always set up a low tech dirt tank if you don't want to mess with CO2. The bacteria and other fauna in your tank will produce enough for plants in lower light situations.
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