I have a buddy who's run tap water direct from the garden hose to the tank. He's been running this way for years, with few problems.
He does not run a lot of the higher light SPS though, and only has minimal coraline algae. But doesn't supplement either...
Water must be pretty good in Concord, CA though.
Due to a remodel, I had to give him my softies for the past couple of months. Now he's gotta get out of the hobby for a while, so in addition to getting my stuff back, I'm getting his, plus his dual 250w halide canopy.
We'll see how nice I can get them looking, just ordered new RO from BFS, and Russ finally got the .35m filter in.
Gonna take a before in his tank, and after in mine after a month or two
If I remember, I'll post some pics. i have no doubt that a lot of tanks can live and thrive even, with tap. And also think that we could possibly going crazy with trying to get ultra-pure water for many tanks that are up.
I wonder if tap with decent filter media, carbons, and a GFO might not be almost as good for non-SPS corals.