You know I have been useing tap water for as long as I can remember, but I have changed my thinking, I set up a reef tank about 6 months ago and was useing tap water and I was haveing all kinds of algee problems, started to use DI water and the problems went away, I think I am hooked, now the problem is I got to start useing it in the other two tanks, I am trying to figure out how many jugs I need to keep around, the lfs is only 5 minutes away, and its only 30 cents a gallon. I think that is the way to go now. Good Luck