You've got that right, Mr. X. Over the course of a day, the water level in that last section will go down significantly due to evaporation. Even if the return pump isn't just sucking air, the splash of the water going over the baffles is going to give you a lot of micro bubbles. I prefer that to a flood, though, and an ATO is pretty easy to set up. I lose about a gallon of water a day to evaporation, so keeping up with that manually just isn't very practical, not to mention when I go out of town.