Yep, extra CO2 in a room can easily cause ph issues in your aquarium. This is a real problem with people that have tightly sealed houses, and/or lots of heavy breathers.

At Natwells in S.A., the guy just grabs your tank, hooks it up, opens the valve, waits a few seconds, closes the valve. Then "checks" the weight by picking up the tank and hefting it for a second, then hands it back. Real high tech and accurate. I once made the mistake of asking him about this and was treated to a sarcastic rant in Spanglish.