Originally Posted by Gregcoyote
You need good airflow to help gas exchange. I would rather top off more often than risk a C02 build up.
Do you think a tank with an overflow, skimmer and a topless sump should be concerned with gas exchange because of a lid? He should be fine with a top considering the high aeration from the rest of the system. I know it's nice to go lidless. I think he'll get plenty of aeration even with a lid.
Maybe try it both ways and see if the lid reduces enough evaporation to make it worthwhile. It'll only take a few days to see a change.