Need at least a couple of inches. Figure out your hardware including the thru-hull, a shut off valve, etc. If you are using aerator pumps you will need a lot more room than if you are just connecting a line that goes to a reversible pump mounted higher up. The Rule 2000 aerator has to screw on and has a port sticking out of the side of it. It can be very difficult to screw onto a shut off valve in a confined space.