Eventually, all of it except for the small amount that the
plants convert to tissue, which is why you have to keep
adding CO2 to maintain levels above just a few ppm.
Consider, if you measure your plants after drying them out
they weight won't be anywhere near the 5 or 10 pounds of
CO2 that you've put into the water over some given period,
not even if you kept all the pruning and trimmings.
In a way, it's a very ineefeicient system, but CO2 is so
cheap, and we're taling about relatively small quanities of
it, compared to other uses, it matters litle.
Scott H.