The issue with tap water is that it contains chlorine, nitrates, metals and other chemicals that can be extremely harmful to aquatic life. Some areas have hard water and limescale is a huge issue for those living in those regions. Flouride is another addition to tap water which varies depending on where you live.
You might hear of reefers saying that they use tap water and while you might get away with using it initially, in the not too distant future you will struggle to keep your tank free from algae. This means regular algae blooms. In addition, coral will not thrive and start to lose their color.
Not only does this totally defeat the object of setting up a reef aquarium in the first place but will set you back a whole lot of time and money. Not to mention disappointment.
Download our ebook on setting up a successful saltwater aquarium here.