Do you have test kits for hardness? I'd recommend taking a couple glasses of tap water and testing them, both right out of the tap, and after it's sat for a day or so. also try one with some of the substrate in it, and check it every couple of days.
I was under the impression that most well water tended to be hard, but that isn't always the case.
Also uncertain how the black diamond would affect pH, it's pretty much just little glass chunks.