From FrugalReef

A wide channel style overflow. Note the nipple at the top w/ venturi valve for sucking air. This needs to be attached to a special pump to keep the siphon clear.

An overflow is a device that allows water on the surface of the tank to "overflow" and fall inside. The idea is to clean any detritus off the top and suck anything that has settled to the top down into the filters in the sump.

There are two principle kinds of overflow. The kind with a wide channel siphon and the kind with a tube siphon.

The wide channel siphon type overflows will collect air bubbles and eventually loose siphon. Usually these designed have a nipple on the top where a special kind of pump can be attached to remove the air. However, these nipples and airlines are known to become commonly clogged leading to a buildup of air bubbles in the channel and a loss of siphon.

The tube siphon style overflow uses a large U bent (usually) acrylic tube. Water velocity in these siphons is much higher, leading to the removal of air bubbles. This style, while looking less sophisticated than the wide channel siphons, actually has a reputation for being far more reliable.