The vent hole in the end-cap prevents a full siphon from forming by letting a controlled amount of air enter the standpipe. This air travels down the drain plumbing and into the sump. This can be anything from a mist of fine air bubbles to a rolling boil of bubbles. If left unchecked the air bubbles can make it back to your return pump, get chopped to micro bubbles and sent to the display area distracting the viewer.

The air bubbles in the sump can be dealt with in many ways:

  • Modern sumps are designed with many baffles which are used to defect air bubbles to the sump surface, reduce the velocity of the water in the sump to allow air bubbles to rise and extend the distance water travels before reaching the return pump to allow the maximum amount of time for air bubbles to rise to the sump surface.
  • Filter socks can be use on the drain line where the water enters the sump.  This not only filters the water (sometimes called a polishing filter) but it does a very good job of trapping air bubbles.  Filter socks tend to be fairly cheap in price and many styles can be cleaned / washed and reused.
  • Large sumps often have a section just for dense macro algae growth known as a refugium.  The macro algae not only removes nutrients from the water but acts like a baffle system deflecting air bubbles to the sump surface.