ph rise

I have two tanks, one 20 gal. and one 75 gal. Both are established tanks and both are reading around 8.2-8.4 PH. I just figured this was what my tap water was, but I checked my tap water and it's around 7.8-8.0. What's causing my PH to rise in my tanks? I know rocks, substrate, decor, etc. can...