First i must say you dont want to cover your tank. You need good air flow above the surface for proper gas exchange. This is your O2
source as Salt water fish get there oxygen from the water and from saturated oxygen not from air bubbles large enough to see. Then your best bio filters , the LR
will convert ammonia and nitrate into nitrogen then released at the surface but if your tank is covered then your O2
is low and bio filters cant work as it should.
Then evap. is normal , my 75 evaps around 6 gallons a week. I use a doser and 5 gallon jug to keep it topped off. But keeping it around the same level is best. As it evaps then salinity changes and in a smaller tank can change fast and the constant up and down isnt wanted. In fact the top off unit is a very easy DIY and help reduce time of maintaining and increase time for viewing.