Eric,

For a fish only saltwater tank tap water can be used. You have to find stuff that makes tap safe. You can find that at any store including walmart. I still do this for my fish only due to the size. I had my tap tested and the water is good. The ro/di unit just makes it better for the corals which are more sensitive. If you are just doing fish then they don't seem to mind as long as you treat the water with an additive to take out the impurities. I've had fish for over 5 years now with no bad effects of using tap water. If you want more info on it just let me know.

Angie