Put a drain hole at the bottom of each grow bed. Then link each grow bed together using a common manifold or pipe below each of your growbeds. Cap one end and on the other end use a 90/elbow upward and create your water level height with the pipe. Use 2 more 90/elbows to come back down, creating a "U" into the fish tank, reduce the size pipe by 1/2 and you should get a good siphon. You can use this method to set whatever level you want as your water height. This is Affnan's common drain, cannot find the exact post. You can check it out here.

http://affnan-aquaponics.blogspot.com/

The beauty of this is you do not need any upstands or stand pipes in the grow beds.

Edit: Found it (http://affnan-aquaponics.blogspot.com/2 ... photo.html)

Siphonshttp://affnan-aquaponics.blogspot.com/2 ... rt-ii.html)

On the left he uses an downspout with a small hole in the bottom to help with systems that cycle their pumps. If you leave your pump on continuously then you do not need this.