NEVER USE TAP WATER! You have a very small tank and the chemicals and elements in regular tap water can build up and become dangerous to the fauna in the tank. RO water is the minimum you should put in your tank. Distilled if you have no choice. If you start your tank off right it will work well for you. The less bad stuff you put in your tank, the less you have to worry about taking out.
Top-off, water changes and starting your tank should always be done with RO water (RO/DI is the best way to go) The units are cheap and last for years. Filter changes are cheap and easy.
Remember you are lighting a nano. Chief might have 250W of light over his tank but you might not need that much power.
Reason #1: More power = more heat. Evaporation in a tank that size may cause problem with salinity in a short period of time.
Reason #2:
ATO (Automatic (water replenishment) Top-off) may be necessary if you do decide to go with higher wattage lighting. Actually you should do this anyway with 150W also. This keeps your tank more stable than just dumping large quantities of water at random times.
Reason #3: Cooling your tank. There are actully a few companies that make small affordable chiller in 1/15th hp just for nano's. A small pump and a place to put the chiller and you are set.
Reason #4: Inhabitants. Some animals don't like alot of light. Figure out what you are going to keep in the long run and find out what it needs to survive.
Combining all of these can be a very inexpensive way to keep your tank at the top of it's game and you happy.
You can have a spectacular nano that you can be proud of and no hassle to you if you plan well. These guys are really hard core on here and sometimes go to overkill for their tanks. But the basis for their individual systems is still the same.
Your goal is stability, stability and stabilty... I cannot stress this enough. Correct husbandry. No impulse buying. Research, research and more research... In a system as small as yours there is really no room for error.
Ask questions, use the search button! and check out the tank threads forum. Get an idea of how you want your tank to look and perform. Sketch out something and look at it. Everything in the hobby doesn't have to be expensive to be good.
Good luck with your tank,
Kasei