i agree, last year when i ran experiments on terrestrials, i saw a 21% increase in growth with the addition of CO2. your tank will be a perfect growth chamber for this. I found that increased humidity during this period slowed the increase I was experiencing. With humidity over 75% my increase in growth was only 16.5%. This was most likely due to an increase in microbial consumption of O2. We cannot neglect the immense amount of O2 molecules needed during cellular respiration. The demand is equally important for glucose to be synthesized and then broken down into ATP. This is the energy that is ultimately used to create more tissues. When done correctly, the addition of CO2 to terrestrials in conjunction with a massive photo boost can do wonders. This will be especially true with the dynamic soil you seem to have going. Dont worry about algae yet imo.
great experiment opportunity, If you want I'll look for my old data and send it thru to ya!