You may want to calibrate your Apex Ph probe. I calibrate mine every 6 to 12 months. BRS recommends every 3 months. As Steve pointed out, CO2 is trapped in the house with us (and cause by us/our pets exhalations). I do pull outside air for the skimmer which helps. You can also use CO2 absorbing material on the input airline to the skimmer as well. Opening the windows this time of year can be chilly. CO2 is heavy and does gravitate to lower elevation, like a basement and lower levels of the home. I monitor Ph and ORP to alert me that something dramatic has changed. i don't chase Ph or ORP. Stability is key.