I just bought 10 lbs of dry ice . good idea to break it up n leave rock chunks on top of the lid or put them in the resevoir and have it pour co2 all over from the inside so it hits the roots.
Roots can and do absorb and use CO2 (this is how Marine plants obtain it). I use CO2 generating tablets in my chili plant hydro, with very good results. I wouldn't use dry ice in a res though, just in the atmosphere.