Depending how much you need to lower it, there are several ways. You could use r/o or distilled water, a buffering substrate, CO2, adding some peat granules in your filter or adding anything that has tannic or humic/fulvic acids will help lower it. It really depends on how much you need to lower it as to which approach to take. I would avoid chemicals, as they aren't very stable and can cause pH crashes. Is this for a soft water species?