Product review (5 stars): Hard Water Helper
With well water as our only option, small-room evaporative humidifiers have been a poor option; they calcify and harden so quickly that wick replacement costs are prohibitive.  The Air-O-Swiss humidifier with this cartridge has made all the difference for us.  We have used the Air-O-Swiss 7145 for over a year and the Demineralization Cartridge has made all the difference.

Even with the extreme hard water we have, the cartridge has done its job and prevented build-up of the many minerals inherent in our water, including an excessive amount of iron.  You can run the 7145/7147 without this cartridge if your water is pristine, but I would otherwise recommend it.