Hey Twitch ,
In mosta my readings Co2 is used in a completely sealed enviroment .

ppm of co2 is generally that of the nute mix . nutes = 1200ppm / co2 = 1200ppm - just an example .

Temps can usually be run about 10 degrees hotter w/o adverse effects also using co2 .

Unless all other aspects of your grow are absolutely perfect , enviromental conditions etc... Co2 isn't nesc. I'm no expert , This info is all 2nd hand from years of reading at dif. sites etc... So I'm not swearing by any of it . My grows rock w/o it btw.

Getting Co2 tanks filled all the time is expensive and draws lotsa attn. to yourself , A propane powerd 1 is dangerous in my eyes as a burn survivor . And yes , you can create a toxic situation for you and your plants if theres too much of it .

Peace
Crispi :jointsmile:

PS . Be careful if u do indeed use blocks of dry ice . It can and will burn the hell out of you in the form of frostbite .