Prevents damage to air source from back siphoning in the event of accidental loss of power, power failure, or "brown-out". Will not allow water to reach valuable air pumps or damage caused from water in the airline. Protects pumps from damaging back-siphoning Prevents water from siphoning through airline Low resistance to airflow Installs anywhere in airline Directions: Simply cut airline at convenient location between air pump and first outlet. Ensure that colored end is furthest from the air pump outlet. Attach airline to both ends of check valve. If air does not flow when pump is turned on, ensure that check valve is properly inserted; reverse if necessary. Fits standard aquarium airline tubing.