One Way Check Valves should be fitted in the air line approximately 150mm (6 inches) from the air pump and are designed with a silicone one-way reed-type valve so air can flow to your equipment in the aquarium without too much restriction. In the event of a power failure, water can potentially syphon back down the air line into your pump, causing damage to the pump and electrical shorting, or even worse drain your tank. These simple one-way check valves stop the water draining out, preventing any water damage to your pump or home. A tiny investment to potentially save costly damage.