Dolomite lime would be a good PH+ buffer, can be top dressed and watered in. Distilled water cannot be tested unless something is added to it such as Epsom Salts, it also it goes acidic setting, 1 tablespoon per gallon of water is good. In flower one wants to raise the PH to 6.5-6.8 for best results in soil (6.2-6.5 in veg)