A said grow room is in a crawl space, and is basically a pit 4ft deep by 5ft by 5ft, but the room itself is 10x10 and only 2ft higher, which makes it like a big counter sunk box within a box...anyways, if I wanted to add co2 could I just put the tank with regulator and all, into the "pit" and have it just fill that small volume ( which is where the plants are ) as opposed to filling the entire volume and wasting co2. Since Co2 is heavy it will just fill pit first and get it up to the required ppm and shut off after that right, this is just my guess and would like some input before I waste any gas. Thanks for any help.

hm...that sounds like a pretty idea situation for CO2 enrichment, a 'natural' hole to use to sort of contain the CO2. Of course...if you're having any sort of ventilation it will lessen the amount of CO2 in there. I guess it depends on a lot of factors, like where the light is, how you're cooling, etc etc, but...yeah, sounds like it might work well.