This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
The above treatment options can be combined to get water with
more or less custom mineral levels. The simplest and most common
approach is to reintroduce proportion of charcoal filtered tap water
downstream of the RO unit to raise hardness and alkalinity to a 30
mg/l range. This is still boiler safe, but greatly improves coffee
taste over straight RO water. It requires no more maintenance than a
straight RO system. Some systems use custom replaceable mineral
cartridges downstream of the RO unit. Aquarium owners use more complex
mineral bed systems to control each key mineral level independently.
Such sytems are extremely high maintenance, requiring frequent water
testing and bed manipulation. Follow the aquarium links at the end to
get more information.