CO2 settles in the absence of air movement and even then, experiments conducted in controlled environments have proven only mild differences in concentration. What you are proposing is impossible.
You're using substandard CO2 supplementation, this is the only problem I see. As well, sealed in an ambiguous word. What is closed to one is open to another.
You say your breathing raises the room to 1800ppm. If you stay in the room does this fluctuate?
What are you using to monitor CO2?
https://www.youtube.com/watch?v=ZAGloLXO9L0
https://pubchem.ncbi.nlm.nih.gov/compound/Dibromine
https://pubchem.ncbi.nlm.nih.gov/compound/oxygen