Wow, people build these things all the time, I built one and it works great.
http://forums.saltwaterfish.com/vb/s...d.php?t=288154
It works like the traps on a sink (and like the commercial CPR overflows). I built mine for about 20 bucks, the key is that you need to be able to remove the air from the very top point of the overflow. Once this is done (and water seals each of the bottom curves) when the water level rises on the side inside the tank it will also rise on the side outside the tank. You don't even need that much of a depth for it to work, though longer runs outside the tank seem to increase velocities in my experience... I've seen this design called a 'velocity loop' before, try Googling that
