my tap'comes at 7.2 and normally after nutes it adjusts on its own to 5.8. i would think because ur starting at such a high ph i would ph the water after it sits out over night or even better if u have the space get a 35 gal garbage can and fill that bitch. leave it over night and then test and adjust. cover it when u achieve ur desired ph and see if it stabilizes. after a day or two if its stable use that water for ur nute water and regular waterings. i would probably drop some 35%+ h202 in there and put an air stone and a pump to prevent pathogens from growing in it. this should keep u in a good water supply for a while unless ur running lots of plants or a huge res that needs refilling.