you use an overflow box that works on a siphon. Even better is to get the corner built in overflow boxes, that don't require the use of a j shaped siphon tube. In the sump is a pump, that sends the water back. Since it is not a closed system (water out and water in are independent of each other), the pump has to be matched to the overflow flow rating. The pump must be set up so that if the overflow/siphon stops, the volume pumped out of the sump will not flood the display tank. Sounds difficult, but it is easy in practice. They sump can hold a large volume of water to increase system volume, and it can hold a very large and efficient wet/dry trickle filter too. Not to mention heaters and other tank things that clutter up the display tank. You could even put animals or plants in the sump, and with the lighting on the sump system and display system out of phase have a remarkably stable system. If you plumb the setup so the sump is in a utility room, you can have holding tanks for water preperation, and do allyour maintenace (except for tank cleaning)away from the display tank.
While I don't have one now, I am a big fan of them.