Product review (3 stars): Seems to keep mildew away, but overpriced at $8+
I have two humidifiers that I have to fill at least once per day. After about 3 weeks I soak the pads in warm water with a little vinegar to dissolve any minerals and restore the pads.  Before this, I also had to soak them in some bleach to get rid of any mildewy smell.  This last time I didn't have to do this last step, and I only add the bacteriostat to the tanks about once every 3 or 4 fills which seems like enough to keep the bacteria/algae t bay.  I just ordered a second bottle.

Update - I originally purchased this for about $4.  It was worth that price, but is now only available for over $8.  I wouldn't purchase it for that price, so dropping the rating as the value is not there.  Instead I'll just add a few drops of bleach to each container, which is significantly cheaper and works just as well.