Wet skimming matching ATO

Had an idea just not sure how to make it happen. I'm currently running an auto top off 26 gal drum that feeds into my sump. I was wondering if I were to do heavy wet skim and had an auto top off to match and would also double as a continuous water change. Is this a foolish idea ? My concern was how to keep my salinity stable since the auto top off which would now be sw doesn't compensate for evaporation