Also touching up on what melosu said,
nitrates are not the only harmful thing in tap water. Just imagine all the chemicals and additives used in tap water that you would never put in your aquarium. There are alot of things in there that coral cannot survive with and stuff that will throw your levels very off balance.
One of main things many people overlook when reefing is that our home aquariums are already nutrient rich and our goal when performing maintainance is removing those excess nutrients. Tap is just adding more fuel to your fire.