-The more CO2 dissolved in water, the lower the pH will be.
-The lower your hardness, the lower your pH will be
-There are several other miscellaneous parameters that could effect pH
You mention in this second point that the lower your hardness, the lower your pH will be. I saw Hoppy say in one post that carbonate hardness (KH) does not affect how much your pH will drop due to CO2 (i.e. 30ppm CO2 always drops the pH by 1.0 no matter if it's in 1dKH water or 15dkH water). Can you elaborate on what you meant when you stated the 2nd point?
Not quite: I believe what I said was that a drop in pH of 1.0 means the ppm of CO2 went up by a factor of 10. So, if the ppm of CO2 in the water started at 3 ppm, the final amount is 30 ppm, no matter what the KH of the water. If I didn't say that, I should have.