CO2 bags are hung high as it is heavier than air … and is far better in sealed rooms ( closed loop ) .
Personally I see no reason for that, as most growers amalgamate towards results, and effectiveness, atleast I do! Extraction fans are commonly set high above, I can easily see a 6" fan sucking out more C02 than it does drop to the floor. There is so much working against this theory. Heat rises, wind turbulence is generally at the canopy, and the extraction fan all go towards disturbance of weighted air downfall.
For me, having C02 pooling at the bottom of a tent and climbing up while being sucked upwards and then sloshed around the plants when it reaches the canopy fans and then further pushed up by the 6" extraction, all makes logical sense to me.
The idea of C02 is to spend as much time possible around the plants, and let them soak it up. I'm sure regulating of PPMs( bigger canisters) can change a whole lot in the tent at any height it is placed, but I'm a hobby grower looking to maximize without breaking the bank. Which is why I don't even recommend CO2 without a dialed in environment, etc... Commerical is where most growers reference too (well I do), because they work hard and diligently to produce the best with the least. Dollars in vs dollars out is something I hold very true, because I want growers to enjoy, and learn, not expect results they can't attain.( So easy to get discouraged when the standard for cannabis is so high now for quality)
Again I don't use C02 often, and my knowledge is rather limited on the subject.