I just purchased an RO/DI unit and did a 50% water change this past Sunday and my PH before was around 7.2. After about a couple hours it was 7.2. The next day it was 6.8. Now....one of two things could have happened. I have DIY CO2. By lowering my KH from 18 to 10, this allowed the buffering capacity of the KH to lower the PH from the addition of CO2 or the RO water has a lower PH. I am leaning on the first. My thread found here
http://www.plantedtank.net/forums/wa...lants-not.html has alot of good info and discussion on KH & PH. I think (and I am no scientist) that when the RO water (that has zero KH) is added and the KH is decreased, and there is the addition of CO2, the PH will drop because there is less buffering capacity in the KH. This is only my opinion from what I have read and my own experience and testing with my new RO/DI unit.