I always run a vent fan and my house CO2 level stays between 450-550 with no change over the last week. Temp is a little lower, but only an average of one degree. The only major change I have done was deep clean my saltwater storage tank last week. I draw two gallons a day from it for daily water change. That’s when the rise started, but all other parameters are stable.Are you keeping any windows open more now that it's cooler here in Florida? We started keeping ours open, and our PH has been getting up to 8.44 at the highest so far.