You need enough to operate, but you don't want it full, because when the power cuts off, your display will drain into the sump for a certain amount. My 120 drains about 5 or 6 gallons into the sump, which is several inches. It is best to fill your display tank to the point it is about to run the overflow, then fill the sump completely, and turn on your pump. Once everything has run for a few minutes, you should mark your sump at the water level, and never fill it past that point when the system is running.