So typically you install a check valve on your return line as close to the pump as possible and when the flow is shut off, the check valve closes and it stops the back flow. HOWEVER, check valves are notoriously unreliable at sealing properly and preventing flow from making its way back to the sump.
Melves Reef came up with sort of a backwards check valve setup that Fin has implemented. In this setup the check valve is installed backwards, above the return height. When the return is running it presses water up against the check valve keeping it closed. The second you turn the return pump off, the pressure drop allows the check valve to open and this lets air into the return breaking the siphon and preventing water from draining back into the sump.
Marc Levenson demonstrates the installation of a backwards / reversed check valve as an siphon break on return plumbing for your aquarium.
www.reef2rainforest.com
The method I use is a little simpler in that I have my return line and instead of a 90° elbow to tie the return line into the retun bulkhead, I will use a T. The top of the T has a slip to thread reducer to reduce the top of the return T down to 1/4". From the top of the T coming out of the reducer I'll have a threaded elbow to RODI fitting. From there it's a simple straight RODI line with a 1/4" valve and then another 90° elbow to bring the line down into the overflow. Once the return is up and running water will flow out of this line and back into the overflow, you can tune the flow to almost a trickle and when you turn the return off, it will instantly allow air into the line breaking the siphon. Here's a very crude drawing of how it's setup.....
Both of these methods are essentially just siphon breaks to prevent excessive back flow into the sump from the Display. You can also drill a hole into your return line itself however these need frequent attention and can be tricky to get right as you need the proper size hole to let enough air in to break the siphon but you don't want too much of a hole that you're letting too much flow out of the hole. Hense the route I took, which takes a 1/4" hole in the line and let's you easily adjust it to match the size you actually need to break the siphon without having to drill multiple holes.
