I recently move to a different county, and the municipal water is extraordinarily hard, with mineral contents pushing the solubility point. It's hard enough that it's causing problems for my RES in the form of mineral build ups on his body. I'm working on reversing the problem, and I'd like to know, more out of curiosity than any actual fear that I'll push the limits of his tolerance, what the idea pH, gH, and kH ranges are for your standard domestic RES.

I realize as relatively hardy animals that they can survive in a broad spectrum of environments, so either a simple xx.x - xx.x range or a median number is fine.