I use RO and its basically like working with coco, It's a blank slate. I use 30 ppm water from 300-400ppm source. 

GH nutes, and haven't had any problems. The water is stored so any chlorides tend to be  disappated then of course whatever any storage tank,  sitting, inviting has in it.

I'm using peroxide in my mix so that's not a problem. 

There isn't reason I couldn't have my tap water analyzed and then create proper mixes with it when it has been rid of the calories.

Lots of folk use rain where its more plentiful.