My wife & I smoke in the house A LOT! I run the air intake for my skimmer outside for a fresh air supply - not a bad idea in general due to how "Air Tight" homes are nowadays. Lots of people/pets in the house can also cause high CO2.

I had horrible problems with Ca levels in my old 125g until I supplied outside air to the skimmer.

BTW - My Powder Blue tang has a two pack a day habit ;)