i would say the main reason would be gas exchange. but even at that point o2 is exchanged more than ever before with the way we activly skim the water in the tanks and the turn over rate of the water through the sump and skimmer. also it is very easy to control temperature now adays with fans and chillers. I believe the glass top is much more neglagable of an issue then it used to be. but i will say this, it is much cheaper to use a canopy because of cooling costs, depending on the size of the tank.