Product review (5 stars): Great system ... and the Mold Lady is right!  ;)
$6 or $7 bucks, lasts for four months or so. We have a HUGE number of indoor plants, especially in the winter ... so, a little extra mold floating around. We use a tiny dash of the anti-mold/bacteria solution ... and we try to run them dry every few fills. Wish we could dry them all the time, but keeping about 40% (35-40 grains) all winter means running these a lot. We have two of these units, plus an excellent auto-controlled Holmes (of course discontinued).

We also run HEPA units, which helps.