Venturi Valve

Venturi valves allow filters to suck in air as well as water so that when the water is returned to the aquarium it returns a flow of bubbles. This can be helpful in keeping the water well oxygenated.

According to Wiki, the Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, Giovanni Battista Venturi.