ang,
Here is the link to Canreef's Library's Chemistry page....
http://www.canreef.com/vbulletin/forumdisplay.php?f=41
IME, when a tank is experiencing pH swings, you need to be concerned about alkalinity most of all, but also magnesium and calcium levels.
Quagmire,
When a room isn't getting enough fresh air, CO2 builds up and you tend to get lower pH.