you must add fresh water to make up for evaporated water in your salt tank.
If you already know the following just iggy it haha.
As your water evaporates from the tank it leaves behind salt. Because of this your salinity in the tank will slowly rise as the water level in your tank drops because of evaporation. In order to keep water in your tank, and keep salinity at a constant lvl you add fresh RODI water to your tank as it evaporates.
DO NOT add salt water for evaporation. If you do you will continually be adding salt to your tank and the salinity will rise too high.
AGAIN, going to state this just IN CASE you dont know (but you probably do)
this is DIFFERENT from water changes. During a water change you drain some of the water directly from your water system and REPLACE IT WITH SALT WATER.
Sorry for the elementary post, but I just didn't want to assume anything.
GL!