Product review (5 stars): It works.
I know it says it's not for ultra sonic machines, but that's what I use it in.  I can't find anything else that works half as well and all the 'home remedy' stuff doesn't work at all.  I've never noticed any 'white residue' with this stuff nor had any adverse reactions or problems.  It does what it says it does, keeps crap from growing in my humidifier.  I use a splash in every tank and it works great, doesn't smell weird, doesn't hurt my machine and does it's job.  Love it.