Product review (5 stars): Works best with DISTILLED WATER
Cartridges are about $4 at Walmart.  They do work - these cartridges worked well when I can't get to the store & have to rely on tap or even bottled drinking or spring water instead of distilled water-but I still get icky smelly pink/reddish slime growth within a couple days if I leave tap water in the humidifier.  I have to clean it every couple days unless  I use distilled water.

The cartridges work best and longest when I use distilled water and when replaced as directed about every 30 days. No smells, icky mold growth, bacteria and reddish slime growing at all when used as directed in conjunction with distilled water and changing the filters timely.  I use a humidifier for my allergies. I don't want to breathe in bacteria or mold and though I pay $$ for it, distilled water, the cleaning cartridge, and filters, keeps my simple CVS cool mist humidifier the cleanest