I wanted to talk about how your water level affects your filtration with an HOB. With the water line above the filter output your filter operates much more efficiently. It's also quieter. By contrast, with the water line below the filter output your filter is basically filtering water that was just filtered. Leaving 3/4 of the tank water unfiltered. For those who say I need the oxygenation, I say get an airstone. For those who say "I like the waterfall sound", I say buy a waterfall decoration. So, for the health of you aquarium fill your tank.