This is a clever method shown to me by a LFS.
Get your U tube, sink it in your sump so its full of water (no bubbles). Rubberband some plastic (sammich baggies work fine) over the ends so its "sealed". Place the U tube back in the overflow. Fill both sides of the overflow box with water and using a stick (or if you hands can fit in) pull off the plastic covering both ends of the U tube. Thats it!
Oh, I was worried about the no-power issue, too. Your overflow has a 2nd chamber in the back. Both sides will remain at a constant level. Your tank will only drain until the water level is below the overflow.
Your overflow will stop draining water when the tank stops feeding water into it. The siphon will not break since your overflow is designed to maintain a certain water level from within itself. This is good because you wont need to restart the siphon when power is restored.
I recommend you seeing what will happen when the power goes out. Just so you know what exactly is going on if/when it actually happens. Like I said, your tank will only drain until the water level goes below your overflow teeth. You only have to worry about your sump being able to hold that much water (on my 29gal tank, only about 3 gals gets drained into the sump). Just make sure your return line doenst siphon water back. A ball-valve should fix that.
