Well here's the thing. Lots of people use tap water and get away with it, with some degree of success. We measure success in reef tanks generally by the growth of the corals. My tank, for instance, now has thousands of dollars worth of successful coral growth and color...
Now what if something happens to suddenly contaminate my water supply? Cracked main taking in sewage or toxins? Contaminated tap water for a 40% water change could potentially wipe out my entire tank of coral. That would effectively end my participation in the hobby, forcing me to sell all my gear for next to nothing because I'm so depressed looking at the empty dead tank. I would lose thousands of dollars and it would take months to part it all out. THAT would be a waste of time and money.
Spending $200 or less on a RO unit could avoid the scenario above. To me, it seems like a pretty good insurance policy.
Will conditioned tap water work? Yup. Will it work every single time for the duration of my keeping a reef tank? Don't know....
