Step 2 – Set up a simple auto top off system. The auto top off system described here does not require any electronics, switches etc. and as a result has a low likelihood of failure. To build this system, mount a float valve in the sump. To do this, drill a ½” hole in a plastic box like the PVC outlet box shown in the picture. Now install the float valve through the ½” hole and connect a line from a RO/DI water source. This line can come straight from the RO/DI unit or from a RO/DI water reservoir. Using a clamp of some sort - clamp the box inside of the sump. In this case, I used a c-clamp. Make sure not to break the glass with the clamp. I used a block of wood to prevent damage. The c-clamp may rust over the years, but the one in my tank is not bad as it never gets wet. I have been running a setup exactly like this on my display tank and my water change tank for over 8 years and have never had a problem. The total cost for this setup is less than $25.