[glampka]

You can use distilled water if you can't get ro/di. When making up water for a water change get yourself a clean bucket dedicated to the tank. Fill with water & add the proper amount of salt. Most brands will tell you how much to add. Put a ph & heater in the bucket & let it mix for 24 hours. After mixing for a few hours, check the salinity with either a refractometer (more exact) or a hydrometer. Add salt or water to get the proper salinity. You can add the water to either the tank or sump. A good way of doing it is to pump the water from the bucket using a ph.
A RO/DI unit can be as inexpensive as $99. It all depends on the model you purchase.