The fitting of a Tetra CV4 Protective Check Valve allows the air pump to be positioned lower than the level of the water (underneath the aquarium). Using one of these will protect the air pump, making sure that no water can 'back-siphon' and enter when it is switched off or during a power failure. This is maintenance-free and fits standard 6mm aquarium airline. Suitable for use with Tetra APS and all other brands of air pumps.