My grow room is 12wx12Lx7h . Bought the titans co2 regulator. A co2 rain tubing, and the autopilot co2 controller. So my grow room is inclosed other than the entry door and a window ac unit. The ac unit has optional switch to allow fresh air in or leave closed. If left closed do I need a 20lb co2 tank or a 50lb tank? My ideal co2 level I want is 1000ppm to 1200ppm so if I set my center zone to 1100 with a 200 fluctuation that would keep me between 1000 and 1200 . But how long would a 20lb tank last I read it would last around 40 to 45 days is that true? Or is 50lb recommended?
I have a 20# and two 5# tanks. One of the 5#'s is attached to the kegerator and not for plants 
I have a 5x10x8 tent reasonably sealed, set to ~1100ppm with cooling = bringing in cold outside air, a 20# tank lasted a few weeks. When it drained, I’d throw a 5# on and go visit Robert’s Oxygen for a 20# exchange, bring it back and hook up the 20#.
It might be that a CO2 supplier isn’t that close to you, but a 20# tank is easily manageable empty or full. A bigger tank would be … unwieldly.
You can scan this thread for CO2 issues & settings:
I haven’t yet, but could go through my notebook and see how long each 20# tank lasted exactly. But I only had to exchange ~3 times over the course of the grow.
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That’s not bad then. I was also looking at a co2 generator but not sure how safe that would be but sounds like a 20lb would work.
I might be misremembering, but I think the CO2 generators work by burning gas (popane?). I don’t think you’d want to do that in a room inside your house… they’re intended for greenhouses.
I have a 12x32 shed and I sectioned off the back 12 feet to give me a 12x12 grow room 
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