I haven't been able set up an R/O system in my current place (condo, no basement, storage room has no water access, tank's on one floor, kitchen's on another, spouse would kill me if I consumed any more of our limited floor space, etc.) so I've been using tap water on my 90 gallon for the past year.
My experience in Calgary has been that sometimes our water is great for a reef tank, and sometimes it's not. It's way better in the winter, but as soon as the snow started melting in the mountains, I had a crazy diatom outbreak, and our tap water was testing pretty high in silicate. Then within a few days of any heavy rain in the summer, I would have small cyano outbreaks. I learned it was better to not change the water if it had been raining a lot and wait for a couple dry days. Unfortunately there weren't many of those last summer.
Unfortunately, the acceptable levels of phosphate and nitrate in drinking water are way higher than what's considered safe for a reef. Most times my tap water test kits don't show any detectable amounts of either in our tap water, but I don't use expensive tests for those ions, mostly because my tank always tests undetectable because any extra is immediately consumed by algae. I can tell there's a nutrient problem with the water I've used based on what's growing in my tank.
My next tank will be R/O only for sure.
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