Nifty siphon overflow device, originally based on an idea from http://waynesworldangelfish.com/automated_water_changes.htm but I changed it to use a rigid acrylic tube.
The tee and elbow set the water level. By sliding it up and down, the tank fill level can be set. You can see the rigit Acrylic siphon tube going from inside the tank to the overflow holder.
This tank is free-standing. I do something slightly different on the other tanks.