I have not tested my GH yet. I still need that kit. I was getting this idea from a quote by Tom Barr. What do you think ? Am I misunderstanding ? I'll leave it out of my schedule if I don't need it.
I generally suggest adding GH booster to raise the GH about 1-2 degrees from what ever it might be in the tap.
That covers both Ca and Mg, some tap has low Ca and high Mg, but generally most has high Ca and lower Mg.
Adding more Mg and Ca causes no issues near as I could ever tell.
Fish or plants.
So that might be added to the above discussion.