CO2 lowers pH.
CO2 + H2O <=> H2CO3 (carbonic acid) <=> H+ (proton) + HCO3- (bicarbonate ion)
your CO2 goes off, CO2 outgases, carbonic acid (which exists in equilibrium) converts to CO2, and outgases as well.
pH is not a good indicator of CO2 though. the pH and kh together tell you a decent amount in the absence of phosphates, but you have phosphate in your tank (i hope). you just have to figure out by the plants and animals whether you should have more, less, or are at the right amount.
Eheim Pimp #296
Canon Pimp #67