what salt water you remove, you put back in. if you have evaporation, you top that off with fresh water. you always check your salt level with a refractometer, or at least a hydrometer.
in other words, you take 5 gallons of salt water out of the tank, you need to put 5 gallons of salt water back in.
your tank evaporates 5 gallons of water, you add 5 gallons of fresh water, because salt does not evaporate. salt creep isn't supposed to happen. whatever splashing you have that is creating the salt spray should be stopped and problem solved.