Use RO/DI freshwater to replace evaporated water because only the water evaporates, not the salt; you’re going to want to do this regularly so you don’t have big swings in salinity due to evaporation.
You’ll also want to do scheduled water changes, where you remove a certain number of gallons of water from your tank and replace it with the same amount of saltwater. When you do this, you should test the salinity of both your tank water and the replacement saltwater with a calibrated refractometer. Sometimes the concentration of saltwater in your tank can gradually creep up or down over time, so you’ll want to use your water changes to correct this and keep it constant. The frequency and amount of water changes is a matter of personal preference. A general rule of thumb is 10% weekly, though.
