Hi
@Victor
As you have a copy of Diana Walstad's book, you may also find it interesting to read "Decomposition in the Sediment as a CO2 Source", which is on page 60 of the Third Edition. The point she makes is that decomposition of organic matter in the substrate releases both carbon dioxide (CO2) and methane (CH4). So, I guess if you are using a soil-based substrate, it will release these gases. If so, then CO2 concentration in the water column could be in excess of that purely due to atmospheric CO2. Indeed, even if not using a soil-based substrate, other organic matter (e.g. detritus) may slightly increase dissolved CO2. Surface agitation of the water and absorption of CO2 by plants will obviously have some bearing on how much CO2 is retained.
JPC