The real deal is with the thermals of the heating plate for the tub.
When you use tap water, (be it city water or well water) as the water is placed into humidification, the mineral solids form and deposit on the bottom of the tub because they can't stay in suspension of the water.
This causes the heating element to require more energy to heat the water to the point it humidifies. Eventually, over time, maybe not today or tomorrow, next week, month or year, the heating element may likely fail.
The other thing is you may find is the mineral deposits could actually pit the plastic and infuse themselves into the plastic making it difficult if not almost impossible to eradicate the minerals from the plastic.
Considering your CPAP is something you expect and rely on to function properly every night, even to the point that your betting your life on it, I think Clint Eastwood's line below is close enough to be appropriate.
I know what you're thinking. "Did he fire six shots or only five?" Well, to tell you the truth, in all this excitement I kind of lost track myself. But being as this is a .44 Magnum, the most powerful handgun in the world, and would blow your head clean off, you've got to ask yourself one question: "Do I feel lucky?" Well, do ya, punk?