Reverse osmosis or distilled water probably would lower it to a point. Any water will lower your tank's pH eventually, but no farther than the pH of the added water.
schizo_fish, you really don't want to go into the path of messing with water chemistry. It is one thing to get water to the pH you want with the right softness, but it is another to maintain them at those levels! Better be stable and slightly wrong in water parameters than to have closer levels but swing all the time...