Oh. Huh. That's not actually all that hard. I was expecting spoon-stands-up-in-coffee-cup hard. You can run calmag up to about 300ppm on base just fine.
Hard water nutes contain lower mineral levels; I would not be surprised if Sulfur was included in that. You'd have to look up the levels- the GH and GH hardwater comparison would help you see better. It's gotta be posted somewhere, lol.

I'm still wondering about those rocks. Just out of curiosity- as your pH drifts up, does your EC drift up as well, over a relatively short period of time (chemical reaction with rocks vs. plants feeding down the nute strength or evap raising it)?