Using tap water will just fuel algae and corals may not like it. Alot depends on what is in the tap water. Does your LFS
sell water? I started with using walmart water from their big ro
machines and had some small algae issues but nothing really bad.
Just keep in mind the water is the most important part of the equation so if you start with good water you have a much better chance to have a thriving tank.
Start with crap water and well you know the answer to that, LOL, crap in crap out.....
Hope that helps!