So you are using the skimmer to depress pH by adding CO2? That seems just opposite of what others are doing. They are trying to strip or reduce CO2 from the air supplied to the skimmer to keep it from depressing their pH.The pH rose in my tank because I used limewater (kalkwasser) for all calcium and alk, and had quite a bit of photosyntheses, some on offset lighting times. Thus, my pH was always above 8.3, but got even higher when the skimmer was not running to draw in more CO2 to offset that level of CO2 consumption.