Ok, gentlemen.....distilled water has been boiled, turned into stream & cooled off (condensed) so it's 100% pure water. It will NOT kill you, but it will suck a few vitamins out of your body when you drink it. The same is true for de-ionized water, but with deionized water, regular tap water is pumped through "ion exchange resin" like someone else posted.....basically, you're removing "salts". RO (aka reverse osmosis) water is regular tap water run through an ULTRA fine "screen", so to speak. The holes are as small as some atoms....SMALL holes, so RO water will work too (ions/salts removed). You can use any of these waters for coolant.

Well water is normally the worst, tap water the second worst & distilled/DI/RO water are all fine. You can buy de-ionized (DI) water in grocery stores too and I use it if some place doesn't carry distilled water...no problem. Those little things you screw onto your faucet or the water pitchers with the filters in therm are making de-ionized water, but I've never tested to see exactly "how de-ionized" the water is after filtering. I just buy gallons for a dollar and don't worry about it.