Ummm the amount of kh(?) in the water is a so called "buffer" it reduces the change in ph, but doesn't stop it.
Adding co2 to your tank WILL cause the ph to drop, but that shouldn't be a problem, especially since most fish living in planted tanks like acidic water anyway.
Tbh, I wouldn't bother messing with the ph once the co2 is in, it would just make it unstable and DANGEROUS.
I am no expert, pretty sure its close to the truth, but could be wrong. =p