Rachel,
I was having consistently low pH for awhile too. I did a test to measure CO2 in the room my tanks were in. Took some water from a bucket of NSW, vigorously aerated it for 10 minutes with an air stone, all the while monitoring pH change with a pH monitor. Did this in the room where the tanks were as well as outside (in January). pH change in the room was very low, about (edit here) .2. pH change outside was something like (edit here) 1.5.
I was astounded at how much CO2 was in our very air-tight condo
After finding out much CO2 and how little O2 we were living with, we decided to open two windows and pretty much kept them open a bit all winter. Now that it is spring, keeping windows open is easy and exchanges the air throughout the whole house, including the tanks.