I've mixed Xtol with deionised water and got a very fine precipitate at the bottles. It doesn't cause me much concern though, I can always filter it. My batch wasn't exposed to light either, so that doesn't seem to be a cause. BTW, real distilled water requires a great amount of energy and 2-3$ for 2 US gals is a bit suspicious.
I don't know what's suspicious about it. I pay about 85 cents for a gallon of steam distilled water at my supermarket.