I just purchased a CO2 scrubber from BRS since I was having the same issues. Come winter time it is hard to not have a build up of co2. Your house is closed up and O2 is no longer entering while CO2 is leaving. I don't like the Kalk option because when your rate of evaporation changes, you will need to mess with your 2-part dosing amounts. Of course, if you don't dose 2-part, it isn't an issue.