The drawings above show an example of how a properly setup sump should operate. In the left drawing the return pump is pumping water back to the display tank, and the display tank overflow is draining back to the sump. The water level in the left side of the sump is set by the water level adjustment plate. The water level in the return pump section is maintained by the ATO’s water level sensor (or manually if no ATO is in use). The height of these water levels MUST leave enough airspace above the water to accommodate drainage from the display tank when the return pump is off. The right drawing shows how you can expect your sump to fill when the return pump is off. We can determine the maximum sump running depth by calculating the volume of water which will drain from the display tank and then subtracting that from the total volume of the sump minus a safety factor. The formula for calculating volume in gallons is (cubic inches)/231 = gallons. Note that for accurate calculations you’ll need to use inside dimensions of your tanks. For example: Given a sump that is 12” wide, 25” long and 12“ deep and a display tank that is 18” wide, 48” long and has a water drop of 1.5“ when the return pump is turned off we can calculate the following: Gallons of water going to the sump is (18 x 48 *1.5)/231 = 1296/231 = 5.61 gallons. In the sump we can calculate the number of gallons in 1” of water with (12 x 25 x 1)/231 = 300/231 = 1.3 gallons. So, the water level in the sump will rise 5.61 gallons/1.3 gallons per inch = 4.32” when the sump is off. The sump is 12” deep and we leave at least 1” of air space for safety so the maximum water depth in the sump when the return pump is running is 11”- 4.32” = 6.68”.