Understanding Overflow Boxes with Diagrams

Here is how an overflow box works and also how to "prime" the tube, understanding how an overflow box works can help out tremendously in expanding or connecting fish tanks.
 Below is a diagram of our speciall modified box (differs slightly from our version), overhanging a regular fish tank or similar body of water inside of a container. Notice the multiple water levels.
 Overflow boxes can prevent your tank from overflowing onto your floor by diverting water into a safety container, they are also obviously used for filtration, they allow a seperate water level in a sump tank or just connecting two tanks with different water levels.
 See overflow in action!! Connecting a wide 75 gallon glass tank with a 29 gallon long with a 16.5 gallon expander added on top!! One of the overflow box's great uses.
 DIY acrylic overflow box, in town parts, $20 to $25 dollars worth of material required. Instructional guide page here.
Getting started, install the box on your tank and adjust accordingly, install the tube, add water to both boxes.
Insert a small air hose into the tube, suck air out or else use a vacuum pump to remove the air and bring water level in the tube up.
The tube is now full, maintaining water levels between the two boxes, water line 2 will stay at this level because of the water resevoir
 It will begin to work as soon as the bridge is made and water level 3 climbs above the gap. Once water flows it will release the rest of this trapped air as bubbles.
 Top view, notice the water resevoir in the diagram below, water will stay in these resevoir's even when the water stops running and when the level goes down.
Rear View of a running box.
Front view of a full and running box.
Even when the unit is lifted out of the tank, it will keep water in the tube allowing it to still function.