The typical way to set up an aquarium with as sump is to have the return pipe running from the return pump, up the side of the tank, over the side and down into the water. However, what you have to be very careful of here is that when the power goes off, that return pipe acts as a siphon, drawing water out of the main display tank down into the sump. And if the volume of water that is siphoned down from the tank into the sump is larger than what the sump can handle, you end up with water all over the floor.

How is the over flowing sump problem solved? There are a number of options, and they are listed here in terms of robustness and success rate.

1. Ensure that the sump can take the drain volume from the main display tank. Couldn't be simplier, just ensure that the sump extra volume available exceeds the volume of tank water that will drain down when the return pump is off.
2. Reduce the volume that drains to the sump by having the return pipe close to the water surface. Closer to the surface it is, less water volume is drained out before the siphon down the return pipe breaks.
3. Reduce the volume that drains to the sump by having a siphon break on the return pipe. This entails drilling a hole in the return piping close to the water surface. This hole will allow air into the return pipe to break the siphon. It has to be sufficiently large so that it allows enough air in and that it wont foul over in a couple of months. It is good practice to regularly clean the hole out.
4. One way flow valves. There are two types, ball and flap. Water can flow in one direction, but when it tries to flow back the other the ball or flap closes, stopping the flow. They do work well if installed correctly. If installed incorrectly they will not close, so take care. The real issue with them is that even minor fouling will cause them to fail. All it takes is a small snail shell and they will not be able to close.
5. Emergency drain line. As a final protection, it isn't a bad idea to have an overflow drain on the sump, so that if for some reason the technique employed doesn't work, then the water is drained to outside the house or down the waster water drains.

And remember, regularly test what ever system is in place on your tank to ensure that it is working. All it takes is a couple of minutes to turn off the return pump, allow things to drain and ensure everything is working as it should. Those few minutes can save you a lot of problems if for some reason it fails and the sump overflows!

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