I guess if O2 drives CO2 out then it'd make sense that PH will incease. I think it's the carbon that decreases PH. I guess I should've paid more attention to science class lol.I always though O2 drives CO2 out, like Neven said, but it doesn't drive the PH up! I always thought that with the O2 being high, it will only stabilize the PH! But I definitely could be wrong, as I'm only assuming!
Maybe test the PH in the water after water change, and test the PH from the tap as well during the water change! Compare it few days later, that'll be a good test to find out!
I'd try that if my tanks weren't planted! =)