As pat8you mentioned, tap water quality really varies from place to place. Some tanks have worked fine with tap water, others have had nothing but trouble. Here's my little story I tell...
I started my tank with filtered tap water, the filter taking out chlorine and heavy metals, and the big chunks. I ran all the tests I could think of on my tap water before putting a drop of it in my tank and they all came back "0" for nitrates, phosphates, etc. So I figure I've got great water.
After about 3 months of my tank being up, I notice that no matter what I do, I just can not get my nitrates in my quarantine tank (10g) down under 10 ppm. No amount of water changes help. So I check my "pure" source water to find out that my once 0.0ppm nitrate water now reads 10.0 ppm nitrates. ???
I call my local municipal water department and ask them about this, because 10.0 ppm nitrates is above the Federal limits on municipal water. They actually sent someone out to my house to get samples, and then ran them through their lab. Thankfully, they got results that were under 0.5ppm. Unfortunately, they couldn't answer the question of why my test kit read 0 one time, but 10 a few months later. They said that obviously the kit was picking up something, but it wasn't the same nitrates they tested for. I emailed the test kit manufacturer, but never heard a peep from them. Interestingly, the same time this "spike" in my source water nitrates happened was about 2-3 weeks after some massive flooding hit our area. I have to believe that the flooding effected the water quality in our area, and the test kit was picking something up... but who knows what.
At that time, I realized that water quality - even in municipal systems - was not a constant thing. It varies all the time. It also made me think about all the stuff that could be in the water that I can't test for. Since I was going with a reef tank, and planned on sinking a fair amount of $$ into corals, I just didn't feel like I could just "wing it" on my source water. I now have a system that will give me 100% pure water - no doubt about it.
Guess my long winded point is that even if your tap water is "perfect" today, it doesn't mean it'll stay that way.
Another part of the story is that part of my filtration system uses PolyFilters. If you're unfamiliar with them, they change colors as they absorb different things. When they absorb copper, they'll turn blue. Copper in a reef tank at high enough levels is a really bad thing. I change out the PolyFilters in the filtration system every year, and they are the most beautiful blue color. I'm not sure how much copper would be going into my tank if I used tap water, but I sure sleep better knowing that I don't have to even worry about it.
