I never used it as a substrate (previous owners did). I did put a little pouch of it in one of my AC filters and the pH in that tank did always come out a bit higher than the others. I have heard that it will to some small degree affect your pH, but I'd ask folks more knowledgeable on the subject than me.
I decided that my tap pH 7.7-7.8 was close enough and don't fiddle with it anymore.