I started out my tank with filtered tap water that at least took out the heavy metals and chlorine... probably some ammonia too. I went that route for about 6 months with no ill effects - water tested "perfect" with all the test kits I had. And of course, after determining my water was "perfect", I stopped testing my source water.
Suddenly nitrates started showing up in my QT no matter how much water changing I did. (Main tank was bigger so it took much longer to show up there.) Long story short, my "perfect" water supply now had nitrates that were showing up more than before. Who knows what else was in there that I couldn't test for.
Point is... municipal water supplies are anything but constant. Their quality can change on a moments notice depending on many many things that are beyond anyone's control. The only way to control it is to take charge of our own purification.
There are plenty of folks that do just great with tap water, properly treated for heavy metals and chlorine/chloramine. It worked fine for me in the beginning, and would probably still be working fine if I was willing to put up with 5-10ppm nitrates in my tank. But they're always running the risk that something in their water system may break down and they won't find out before they've contaminated their tank. I know in Western WA there are normally "no drink/boil your water" notices that come out for various areas several times a year when something goes haywire at a treatment plant.
I now use filtered water run through DI
resin to make sure my water is the same from batch to batch - no doubts in my mind.
Just my nickels worth.