From The Aquarium Wiki
What are they?
These come in a variety of shapes, often just blue or white, to enable you to put the final touches to your layout.
Typically they are designed for 6mm tubing and are male-male extension, male-male right angle extension, single male to dual male extension.
You can also get small clamps to reduce the flow of air into a tube (though beware that this doesn't cause back pressure as this can damage the internal air pump diaphragm long term as well as increasing noise from the pump.
- Tip: DIY and Gardening stores often carry small tube extensions for connecting thin watering tubes. Whilst some may be only 4mm in diameter and therefore too narrow, there are 7mm extensions which equally do the job and can be cheaper than those in an aquarium shop. (Of the airline valves shown here, only the blue one is sold in aquarium shops.)