I should push the heater down so it stays submerged during my water changes. Currently it's near the top of the compartment. I haven't looked at the back compartments once the water has been drained but the design leads me to believe the level goes up and down the same amount as the display area. For some reason the design has a drain grill at the top and bottom of the tank and it's the bottom grill that causes the sump area to drop the same amount as the display. I'm sure there is a reason for this design but I don't understand it. At any pushing the heater down would alleviate future "duh" moments. LOLshouldn't the heater stay submerged during a AIO waterchange? are you taking water out from the back compartments?