I used straight tap water for 2 months when I started the tank up. Had hair algae all over the place. I purchased an API Tap Water Filter for something like $20 bucks. I have used it ever since and I no longer have any hair algea in my tank. The water is clear and it tests out perfect for Nitrates, Nitrites, Phosphates & Ammonia (0 across the board).
I did not get a RO system for 2 reasons. First, I did not want to set it up in the upstairs guest bathroom (where I do all of my mixing) and second because I live in a "high desert" and I did not want to waste water (RO/DI systems create a lot of waste water).
I highly recommend this unit and it can be purchased at the 2 doctors site.