in todays world,...right or wrong,...the census is to leave your saltwater tank without a top,...the reasoning behind this is that your tanks water needs a way to expell built up gasses thru the water surface.,kinda like keeping all your windows in your house closed sooner or later the air will become very stale and toxic .
the down side to this is that your tank will lose more water thru evaporation,..thats where the automatic top offs ( ato ) come into play,...as your saltwater evaporates , the water evaporates but leaves the salt behind making your tanks water more saltier,...the automatic top offs replaces that water with the same amount of clean fresh water thus keeping your tanks salinity stable