This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
There are anti-scaling treatments available that do not rely
on lowering hardness levels. However none can be recommended.
The first uses crystalline polyphosphate additives. These
additives are FDA approved for drinking water, and some companies do
sell them as espresso machine water treatments. They work because the
polyphosphates form a crystal core to which the scale clings rather
than depositing on metals. These particles remain suspended in the
water. Barry says this treatment is worse for coffee taste than simple
softening, and this makes sense. The limescale encrusted polyphosphate
crystals will lodge in the grinds and interfere with the extraction. I
have found one Austrian espresso machine dealer who sells a
polyphosphate cake to be dropped into the tank and changed annually.
I'd welcome any reports about their efficacy and effect on taste.
A second approach electrifies the water with a low powered,
radio frequency alternating current. No authority I've found endorses
this method except for the tongue in cheek Wessex water board, which
comments that "some of our more cherished customers like it." Perhaps
they have proponents of Kirlian photography as customers, who,
according to their web sites, think this also imbues water with a sort
of life force.
Finally, some people on Kaffee-Netz keep their water mildly
acidic using lemons or citric acid. If the water's pH is kept below
about 6.3, this will work to eliminate scaling in moderately hard
water. This is in essence a simple mineral bed treatment. It will stop
scaling, but the effect on coffee taste may be worse than softened
water. The required level is about 1/4 to 1/3 teaspoon of citric acid