Product review (5 stars): Very effective
Every time I refilled my humidifer tank, I added a splash of "BestAir 3BT, Original BT Humidifier Bacteriostatic Water Treatment" and it kept it clean all winter. Through the winter (I only need to use it from November-March), I inspected it weekly for contamination, yet it remained pristine. I never had to scrub off any weird mold or scum or other growths that would typically thrive inside a humidifier tank.  The filter remained clean as well and did not need replacement.

As a side thought, I am a little paranoid that this chemical may not be 100% safe to breathe but on the other hand, it's probably no more dangerous than breathing in the mold and bacteria that it prevents. Pick your poison, I guess...