A HOB (Hang On Back) overflow or weir is a device used to drain water from the aquarium safely into a sump.
You need to prepare yourself, as this is about 30 seconds of your life that will get your heart beating faster than you'd expect.
- Get a piece of flexible airline tubing, about 30" long.
- Get a large cup filled with RO or tank water.
- Get a second person to be in charge of turning the pump off if needed.
Thread the piece of tubing into your overflow box so that the end is at the top (apex) of your U-tube (or U chamber if you have a wier). The other end goes in your mouth.
Have your assistant turn on the return pump. Water from your sump will pour into your tank and the water level will rise. Do NOT panic, you have about 30 seconds to do the following, which is plenty of time.
Take your cup of water, and pour it in the OUTER section of your overflow box. Without delay, suck out the air out of the U-tube until it is full of water. Bite down or pinch the tubing firmly, so that no air can go back into the U-tube.
Holding the U-tube in place with one hand, completely pull out the flexible tubing gently.
Any trapped air in the U-tube should be washed through by the flow of water.
If for some reason you aren't able to get this worked out the first time, tell your assistant to turn off the pump, and water will drain down via the return plumbing. Once things level out, try again as explained above.
If you have two or more U-tubes, just suck the air out of each tube, one by one systematically.
When you are done and all is working, turn off the pump. Carefully inspect the overflow box to make sure the siphon has not broken and that the U-tube(s) is(are) still full of water. If the siphon is not broken, all is set up correctly. Restart your system and the overflow box will continue as before.
If it worked correctly, you can be assured that all will be fine in the event of a power failure and subsequent return of power to your system - even when you are away.
To set the correct water level in your sump, please refer to my What Is A Sump to prevent your system from ever overflowing.