Straight out the tap is fine for your rats. If you and your family can drink it, for heaven's sakes so can the rodents LOL. Distilled water isn't good for anyone.

http://www.ionizers.org/distilled_water.html

I always laugh at the "special" water they sell at the pet store for dogs. This for something that will happily drink from my toilet bowl and licks it own butt???? How silly LOL.