Keep your kh around 4-5 degrees. That'll give you sufficient buffering for co2 injection. Ph readings can be very deceptive with water that has a low buffering capacity. Use kh values instead of ph to set buffering.
FILSTAR Pimp #2
75 gal heavily planted,50/50 Black beauty,Eco-complete substrate, Pressurised CO2 with solenoid, ph controller, AB Reactor 1000, 330 watts 9325K GE PC lighting,Ehiem Liquidoser, 2-Filstar xp3 canisters.