Good advice so far!
I'll just point out that if you're using tap water, there may be nutrients in the tap water that can feed ugly algae, so unfortunately I think that when you turn your light on, you're likely to get some algae growing. I can't say for sure because some areas have very, very good tap water, but I believe most locations do not, and most saltwater hobbyists use an RODI filter to make purified freshwater to use for making saltwater and for topping up evaporated freshwater.
If you don't have an RODI filter, you may be able to purchase water from your LFS, or consider using galloons of distilled water that can be found at any grocery store, pharmacy, or your local Mega-Lo-Mart for a little over a dollar a gallon.
Good luck!