I used R.O water to calibrate my refractometer, it's the same water I use to make my SW. Put 2 drops (not SW) on lens and then dialed down to zero, is this correct?
i dont know how much i would trust grocery store di water in my opinon i dont know if i would i still think there are some chemicals that are not good for ur tank in there.
RO water (I assume that it is home made RO) will work just fine. Distilled water will also work, but stay away from bottled beverage water (some contain lots of minerals). Tap water works too.