Excel doesn't alter pH.
The theory behind it is plants don't really want the co2, they want the carbon, so you add a product that they can get the molecule from.
Despite the nifty diagram, I have found that very few plants respond to Excel alone. There's just no comparison when you run two identical tanks - one with pressurized co2, and one with Excel only.
It's really ment to be used in addition to
CO2 injection. I myself use it in my pressurized CO2 setup.