Ideally you use distilled water in a humidifier, which should really help to keep it clean. But that can be a pain. I add a small amount of bleach every other

Ideally you use distilled water in a humidifier, which should really help to keep it clean. But that can be a pain. I add a small amount of bleach every other time I fill the tank (a few drops - if you can smell it you've added too much) and once a week I empty it out entirely, wipe it down, and let it dry completely. And, of course, if I'm turning it off for longer than 12 hours or so, I empty it out.