I have a very good reason for not adding Ca and Mg. Coffee beans are laced with Ca and Mg already, dramatically more than even our ultra hard water in this area would provide for espresso. (For other forms of brewed coffee where there is ten or more times as much water, one can get to the point with hard water where a significant portion of the Ca and Mg can be coming from the polluted water. This becomes a different situation.)

What we really need is a very constant water composition that does not precipitate anything and that keeps a constant pH, because many of the thousand or so compounds in coffee are weak acid or bases. Precipitation of solids in the boiler not only damages boilers and fittings, it also results in water that is constantly changing composition. Municipal water supplies are often amazingly erratic. It seems sensible to put as little additives in the water as possible because we are making coffee drinks, not water pollutant cocktails. I strive to make the product be from the coffee as much as feasible!

Many, however, become accustomed to having all sorts of pollutants in their water, and like it that way!

When you add Ca and/or Mg to pure deionized water and then brew coffee with it you find the resultant coffee has a markedly different taste than the same coffee if it had been brewed with the pure deionized water. If you find the taste of the coffee brewed with the pure deionized water objectionable then adjust temperature, grind, dose to water ratio to reach the taste you desire. There is no real justification to be building a chemical soup to use to brew coffee. Depending on how long the pure deionized water has aged will determine if it requires a Ph buffer to protect the brewing equipment.