He is right but one thing that was not mentioned is he has a mini split hanging in the tent so there is very little leaking out of the tent. Also in you consider CO2 is heavy and wants to settle at the floor it might even be less leakage.Alright since this is a thread about CO2, lets clear some things up.
Atmospheric CO2 is about 400PPM.
Desired is 1200 (for the sake of this post, YMMV)
@dreamnfox has a 4x4x8 (not sure on the height but close enough)
So 128 cubic feet approx. It will actually be less, so this is conservative.
1 pound of pure CO2 is 8.7 Cubic Feet.
You want it to be 800PPM increase, or .0008X
Hopefully you see where I am going with this, we are dealing with minuscule amounts of CO2. CO2 makes up .04% at atmospheric and we want to go to .12%. A small leak in the tent will leak 99.88% Nitrogen, Oxygen and other gasses and only .08% of the CO2 you are injecting. And only then if you are pressurizing the tent.
When my CO2 controller asks for a shot of CO2 from the tank, it runs about 5 or 6 seconds, and does that maybe 3X per hour. It is not a continual usage, and at night is zero. Size and stage of growth also a factor.
FWIW my basement sits at about 525PPM when I am not growing, and gets up to 575 or so when I am running both 4x8s and injecting. So yes, leaks but the cost is pretty much irrelevant if you are paying $41 for 870 Cubic Feet of it.
I just don't want anyone to be scared away from CO2 if you have a good reason to use it. It can be a game changer in terms of yields and growth rate. The cost is a tiny fraction of the ultimate reward, and can even SAVE you money since you grow faster so you get the same amount of end product while running your lights, etc for less time.