Stable is far better than exact. A range of 6 to 7.5 sounds a bit out of whack to me. I have several different types of fish, from different places in the world, all living in water at a pH of 6.8. I'm sure it's not exactly each one's preferred, but they seem to be doing just fine.
You're out of luck. Even water is a chemical . . . pH is a measurement of the concentration of a chemical - hydronium ions.
As the first reply said - no real need to lower it. If you have high light you'll want to add CO2 (a chemical) which will also lower the pH. One type of pH down uses an acid to destroy the KH (bicarbonate ions), resulting in a lower pH.
Why do you want to lower your PH?... Fish can adapt to various ranges of Ph, as long as the change is slow. I would advise not to let your Ph drop below 6.
Is your Ph staying stable or is it going up and down?... If it is going from 6 to 7.5 during the course of the day then something is not quite right within your tank. As already mention a stable Ph is the best.