Overflow question
 

How does the tube on the top of the overflow work?

Its a syphon hang-on.

Thank you,

The u-tube is where the water travels up from your tank, over the side, then down.

The u-tube needs to be "primed", and each end of the u-tube when properlly designed, should be submerged in water, so fill up those areas of the oveflow, and then stick an airline into the u-tube and start sucking out the air, this will prime the siphon.

Now you can turn on your return pump, and water should flow into the overflow box, up the tube, into the external box of the overflow then down into your sump.

Keep the flow rate high enough through the u-tube to discourage bubbles from forming, but not so high that the u-tube cannot keep up.

Also keep it 100% clean of algae, a wire brush works well for this.

Some u-tubes have an air line installed into the top for easy priming and air-bubble evacuation.

What kind of overflow are you talking about? U-Tube or C-Siphon style.

If its a C-Siphon you connect a powerhead or aqualifter pump to it and it sucks out excess air trapped in the siphon preventing failure. I think most u-tubes dont have this feature.

You can do the same thing by installing an air-line into the top of the u-tube. But you are right, most do not come with this, and they are not necessary like they are with c-style ones that more prone to collect air and fail.

I assume his is a u-tube, seeing as he asked how the tube on top works.