Product review (5 stars): It's like the whole printer/toner cartridges thing-- they get you on ...
Oh man, I was so pissed to realize how expensive humidifier filters were after I bought a big 9gal/day Homes humidifier.  It's like the whole printer/toner cartridges thing-- they get you on the replacement stuff!  I've watched paper humidifiers will grow browner and browner, and then black and green!  Gross!

This solution really makes a big impact on how long my humidifier lasts.  Usually the regular lasts only about 1 or 1.5 months.  Have been using this with humidifier filter for 2 months now and the filter is mostly pristine.  Awesome.