Why do you want to lower your pH? Do you have really sensitive fish?
I wish my pH was at 7.8 lol My pH is at 8.1, yet i have no problems at all with fish.
In my Opinion, GH/KH is way more important than pH.
My fish stock contains Harlequin rasboras and zebra danios, both fish that are supposed to be kept in lower pH tanks. but are doing extremely well in my aquarium. I pay more attention to GH and KH.
As long as your pH remains stable, and don't plan on keeping sensitive fish you'll be fine. You'll do more damage trying to lower the pH, causing pH swings.
Relax, everything will be fine.
The Fraternity of Dirt #169