I use boric acid in my humidifier. Boric acid is an antiseptic so it will retard the growth of micro organisms. It is a solid at room temperature and is water soluble. Since it is a solid it does not evaporate with the water and thus remains in the humidifier and filter during use. When you add new water it mixes in the new water so it keeps working.

I use about a tablespoon in my humidifier.

Boric acid is available at many pharmacies and in the nursery dept. of many home improvement stores (it is also used to kill roaches and silverfish).