One more thing, you will of course after you set your sump capacity where you want it, need to remove water to add rock, sand, and whatever else in the tank, since that displaces water. Another thing, if your overflow loses siphon when you shut the pump off, no worries, just make sure the tank is full as you can get it without it flooding over, then add water directly to the sump till its 1 or 2 inches from the top.