How much water do you need to change? Distilled water from the supermarket isn't usually much more than a dollar or so a gallon. If you just need to change a couple of gallons for right now, I would trust the distilled water over using unfiltered well water. I'm on well water and we were told when we originally built our house that our water was some of the best in the area the plumber had seen- but even so, a TDS meter shows a reading of 70ppm, and I really don't want to take a chance at what makes up that 70ppms. Some people claim that distilled water can have copper from the distilling coils they use in making distilled water, but I have never known anyone personally who has had a problem with copper from distilled water. In fact, I would worry more about copper from my own plumbing if the water is not filtered.