The site referenced is a good one.
Quote 1: The CO2/KH/PH conversion tables or formulae are based on carbonate/bicarbonate buffering (that present in most natural waters), not phosphate or others. CO2 and the carbonate/bicarbonate buffer system interact directly. If other buffering is present, the pH and KH values will be different from carbonate/bicarbonate only, and the table or formulae cannot be used to read CO2.
Quote 2: The equilibrium with atmospheric CO2 is approx. 2-5 ppm CO2 (varying with elevation above sea level). The lower reading there is from high in the Rockies - if you live higher, you could get lower. On this one I differ from Chuck's conclusions a bit. I suspect (but have not asked) that he is giving an average figure, assuming certain levels of water disturbance from HOB filters, etc., and moderate fish loads. With filtration handled carefully, good subsurface water circulation but no surface disruption, and still with moderate fish loads, you can get above 5ppm CO2, certainly at the end of the dark cycle. I agree with him that 20ppm CO2 is unreachable with such techniques. Adding more fish will certainly increase CO2 a bit, but will increase NH4, phosphates, general pollution even more, and decrease O2 to levels requiring added aeration, blowing off CO2 - it quickly becomes a lose/lose situation. Overcrowded plant tanks are a no win situation from what I've seen on the boards.
HTH