Product review (5 stars): Works well, but the suggested amount is, in my opinion, more than is needed
Haven't had any algae/mold/etc.  Important info: I wouldn't use it as often as directed.  They say about a cap full for every 1 gallon.  If you run your humidifier continuously, as I do, then you are going to fly through a bottle.  (So much so that it would probably just be cheaper not to buy this product at all and to just buy new filters.  Buying the new filters would also provide more consistently efficient evaporation due to less buildup.)  Once I realized how fast I was going through bottles, however, I started using a cap full amount around every 5 gallons.  The latter frequency has still maintained completely clean filters.  My hunch is that if you use a sufficient amount of this product when you start with new filters, then the antimicrobial chemicals will build up on the filters and thus it will not take nearly as much product to maintain their cleanliness.  (Only a guess, of course.) The filters only last around a month before there is too much mineral build-up, so initial and then intermittent use of this product has been enough to keep my filters clean.