First I would change your water supply. Try using RO or at least some type of good quality filtered water. Even if you need to buy bottled water once just to see if that changes the issues. Second, the cal mag I believe could also raise the pH. That combined with using city water could be a problem. Again you could ask GH about cal mag raising pH. Also If your water's pH is that high before adding the
nutrients, you should adjust the pH before adding the nutrients, as well as checking it after. I'm not familiar with using earth stone growing media, but it may be recommended to be pH balanced before use. I would test it. Try adjusting the pH of about half a gallon of plan water to 6.0. Then taking some of the growing media and placing it in the water. Then check it a couple of times a day for a couple of days to see if it alters the pH, and if so how fast.
P.S.
You may ask GH if the nutrients you are using have pH stabilizers in it. If not it will naturaly fluctuate more than ones that do.