My 90 was run on tap water in Calgary because I had no where to put an R/O system in my last condo. It's true that there is the chance a freak event could happen and tap water could nuke your tank, but to be honest, if something like sewage or untreated water, or some sort of heavy metal accidentally gets in to the municipal water supply I'm of the opinion that our tanks should be the least of our worries. I drink that water.
In reality, those sorts of events are probably not something you need to worry about on a day to day basis, and as many people on here have demonstrated, tap water is generally fine. However, just because it's probably not going to nuke your tank in a one time freak accident doesn't mean that tap water is 'ideal'. Not for the big bad fears, but for day to day annoyances. There is significant seasonal variability in the quality of our water, it's always top notch from a human consumption point of view, but from a reef tank perspective, it could be good sometimes, and not so good others. I'm not sure where edmonton's main water source comes from, or what regions it travels through to get there, but Calgary's drinking water starts in the mountains and thankfully travels a pretty short distance before it gets to us. As a plus, Calgary's mountain water is actually pretty hard, which can be beneficial for a reef tank. As a minus, the mineral content isn't always constant, so reef specific salts would sometimes lead to solutions with sky high alkalinity when mixed with tap water, and other times it would be spot on.
Also, there was a distinct seasonal appearance of certain types of algae. In the spring I noticed I was constantly battling diatoms, I assume from all the silicate that the snow melt was carrying down from the mountains. In the summer, I learned to not do a water change within a few days of a major rainstorm (which made June a very tricky month), as cyano would explode if I used post-rainstorm tap water. Again, I'm making an assumption, but I think it's because right after a heavy rain the nutrient content of our tap water probably spikes.
So yes, it can work, it's probably not going to kill your tank, but if you end up with mysterious algae problems or unstable water parameters, you're never going to be able to totally isolate any one specific variable or cause with the wild card of tap water in the mix.
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