Wait, using supplemental CO2 or just "regular air?" If regular air, then 300PPM Co2 would get used up yes, and should be replenished. Only way to tell how often is to hook up a co2 monitor and watch it for a couple hours. THat's how I learned my building doesn't seal well enough to use CO2, I burned a pound of propane in 1 hour and it leaked out in minutes, not good!