if you want a top and are worried about the gas exchange, get a sheet of acrylic or plexiglass and cut a hole in one corner to put a small pc
fan and a hole in the other for air to circulate. the thing with having lids on saltwater, is it causes some pretty bad salt creep but it also helps by reducing the evaporation rate. you would probably need to clean it once a week or it will start to let less and less light through. glass also can reduce light up to 20% and plexiglass/acrylic will only reduce it about 3-5%. do you dust your house often? do you have a protein skimmer? is there enough surface agitation? it could be something completely different than dust, but if there is dust settling on the surface than you definitely don't have enough surface agitation. if its building a layer on the surface, that can also significantly reduce your Dissolved Oxygen level which is not good.