The level in the DT should be determined by the level of the surface overflow unless the drain cannot handle the flow in which case you have a bigger issue. In general, the drain should handle the amount of water that flows over the overflow. I have a modified "BeanAnimal" setup with a cost-to-coast overflow: In my setup, 40" of the 48" inch length of the back of my tank is an overflow. It pulls a long thin stream of water. This drops down into a weir with three drains in it. One is a siphon durso that allows no air. The other is an open channel durso (has a hole in the top) that does allow air. The third is just an open tube that is cut so that it can flow the full return volume without overflowing the tank. By adjusting the gate valve in the siphon durso I can match the return pump. If the gate is open too much, then the siphon durso draws more water than the overflow feeds into the weir. This causes the water level to oscillate from the durso entry tube to just below this tube. When it goes below, air does get into the siphon durso and it makes considerable noise and bubbles go down to the sump. If the gate valve is closed too much, then the siphon is not handling all of the overflow so the rest goes down the open channel durso and makes a fair amount of noise. At the match spot, a little water trickles down the open channel without making any noise. There is a little bit of range on this since there is a range of flows where the open channel's flow is laminar down the sides of the tube. If both the siphon and open channel durso get clogged, all of the water goes down the emergency open pipe and this makes a honking lot of noise so I know I have an issue. ANyway, TMI, but that is how I have mine setup. No noise whatsoever, and no bubbles at all. The only noise comes from the slight hum of the pumps and the skimmer. I have to turn off my computer to hear the tank since the computer fan is noisier that the 120G tank.