I never have any stubborn water stains on my reptile tanks but the newt tank that I received from a friend was extremely stained by tap water.

I've always only used water that I buy from a grocery store. I go for distilled water that's been filtered with reverse osmosis so that it is mineral free. $0.58 per gallon isn't too bad since I only use 2 gallons per week for my tropical reptiles. Water additives like Repti-Safe cannot remove the minerals and metals from tap water. Additives are needed for distilled water when used with amphibians or fish. Adding some vitamin D3 and electrolytes to the water for reptiles is a great way to ensure that they are getting enough.