Simple 4 mm air line non return valve, cylindrical item at the top of the group image.

Helps to prevent back siphoning of water in to your air pump

Simply slide air line over each end,the correct way round and it will allow air to pass through to the airstone,but not water back up to the airpump should it be switched off or in the event of a power cut

Great value

We advise changing them regularly to prevent failure from age or over use etc