YOu could cover yoru tank with a tight fitting lid but its not recommended. Most of us salties run open top tanks to promote evaporation. Evaporation aids in keeping the tank cool as well as aids in gas exchange.
Autotop offs work by the use of a float switch. When the float drops to a preset level it activates a pump or is gravity fed and it adds water to the return chamber till the float rises to a predetermined cut off level.
The autotop off will just keep you from having to add topoff water daily.