Soil sequesters a tremendous amount of CO2, but soil minerals doesn't produce it--it's produced by microbes living in the soil. Also, as soon as you disturb the soil, the CO2 gasses escape. So what I'm saying is that your soil that you put in your tank is going to have little to no CO2 in it--but don't get too hung up on that. There are multiple ways in which you can supplement or supply CO2 to an aquarium. Having CO2 in the soil doesn't do a plant much good anyway, they cannot absorb CO2 through their roots, only stems and leaves.
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