Best way to raise just KH and PH

Running a planted 15 gallon cube and the PH is lower than my API test can read or is just at 6.0. My KH is at a whopping 0. My GH is somewhere around 5 dGH. I’ve heard a lot about wonder shells and seachem. What is the easiest to use and maintain. Also if my tap water is so low in minerals how would I go about doing a water change without the new water shocking the shrimp?