A venturi is a hollow tube that is cylindrical,with a unique shape through which water is pumped.
The Venturi is a very popular, and simple, method of introducing air into the pond system without having to connect any electricity supply.

These Venturi simply connect on the inside of the pond inlet return to the pond, and as the water is pumped back to the pond it is passed through the Venturi which also draws in air from the atmosphere as it passes through.

Advantages of this type of aeration are:
Cheap to run(if you can use an exsisting pond pump).
No expensive hardware to purchase
No additional electrical insatllations