Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
CO2 lowers the pH of a tank because some of it combines with water to form carbonic acid.
Excel is often thought of as "liquid CO2", but that's really incorrect. It supplies a limited amount of carbon to plants through an alternate means, and doesn't produce CO2 at all. So you will not see a significant pH drop. Nor will it affect a drop checker.
If you have a good pH meter, you might see a very small pH drop. That's because Excel is acidic. But it's too weak an acid, and the dosage too small, to detect the change with a pH test kit.