I've never looked into FloraMax, but just saw a image of the bag, and it says "Will Not Raise pH".
So, double check if it really is the substrate raising the pH.
Maybe it's just your tap water has quite a bit of dissolved co2 causing the pH to be 7.5, but after degassing most of the dissolved co2, it would be 8.2? Seems pretty drastic though.
If it really is the substrate, see if it contains carbonate or bicarbonate sources (which would raise KH), such as Calcium Carbonate (crushed corals, oyster shells, eggshells, aragonite, etc).
If it is the substrate raising the pH, it would be pretty hard to combat the alkalinity buffering. Would be less hassle/much easier to use a different substrate (inert).