Answered a year ago here:
Purified or tap water?
Basically, PURE water is a detoxifier by purpose, but a LEACHER by default. Pure water has a better osmotic potential than your cells. You know, Grade 9 Science where things go from hi-concentration to low-concentration? Well minerals are ESSENTIAL to an animal's (and plant's) daily processes. We NEED Ca, Mg, Fe, Zn, K, Na, etc etc etc. Drink water that takes that stuff out and you're in for a world of hurt. Great to get rid of free radicals and other poisons that the average N. American eats daily. But reptiles don't have those toxins, so what does purified water take from them? All the ions necessary to complete every process in their bodies!!
Chemists call it "deionized water". And what does deionized water want to do more than anything else? It wants to ionize itself.