The Free Online Aquaculture Dictionary

Air Lock

In the top diagram, the outlet into the tank is above the water and any air in the pipe can escape.

In the lower left diagram, the outlet is below the water surface. Air in the pipe cannot escape through the outlet and builds up in the pipe, eventually sealing the pipe and preventing further flow of water.

In the lower right diagram an air vent or air purger is fitted which allows trapped air to escape, preventing it's build up in the pipe.