hmmm.... I am at a college in Wisconsin and between 2 of my friends and me we have: 75 gallon salt, 55 gallon salt, 46 gallon salt, 2x30 gallon salt, 20 gallon salt, 30 gallon fresh, 20 gallon fresh, and 10 gallon fresh. We have been using the tap water here at school for all of our fish tanks and so far have a great sucess ratio. Right now I have one sick fish but that is it for the year. Tap water has been fine.