On startup the skimmer will need to be primed. To do this turn off the air tap, fill the body of the skimmer with water from the tank, using a jug, and then plug the pump in. Water should then rise up the inlet tube and start to flow through the skimmer. At this point the air tap can slowly be opened and set.

For further details on priming the skimmer click here for the user manual

If the skimmer fails to prime at this point check the inlet pipe for any cracks or damage and that the orange tap on the air inlet is shutting fully. If air is getting in to the inlet pipe at this stage a vaccum will not form and the skimmer will not be able to prime.