I think if I was determined to use the tank water, I would at least boil it.
I use boiled water. Was told by Dr's at the sleep clinic at local hospital to do so. All the bottled distilled water i've seen say NOT FOR HUMAN CONSUMPTION
I'm not a Doctor but a fan of The Doctor. any views,comments etc are my own
I use distilled water from a local grocery/general store. It is produced by a local company whose main business is producing and delivering water in 5-gallon carboys for use in office coolers. The 1-gallon jugs I buy are marked "dispensed in sterile containers". I dump and wash the water tub daily, and don't concern myself with cost, as the price is around 70 cents/gallon. A single gallon will provide a lot of refills.
PAPing in NE Ohio, with a pack of Cairn terriers
I use Pureau filtered water (comes in a wine-cask style of packaging) from the supermarket. I used to use the 'boiled then cooled' water technique for years, but recently found that this was a good alternative. It works better for me, at any rate, as I find there's less discolouration of the plastic in the water tub.