what size tank and what temp is it running. What type of lights..
/salinity are you looking to get? Most keep there tanks at 1.025 for reefs and between .020-.025 for fowlrs..
The only time you should add new salt water is when you do a water change.
And yes its normal to have your Salinity raise as the water evaporates, salt does not evaporate so its left behind. Just keep up on top offs and you'll be fine.
I have a auto top off so I don't have to worry about salinity swings.
In smaller tanks it does not take much evaporation to get a swing in salinity.