Features: 1.Made of high quality plastic material, good stability and durability. 2.The water flow is reversed due to inside and outside pressure difference when oxygen filling pump is suddenly stopped, and the oxygen-filled pump is damaged and the water in the cylinder overflows. Prevent the water in the fish tank from flowing back into the air pump and burn the air pump. 3.Prevent water backflow from damaging the front-end air pump or other product equipment when the air pump is placed below the water surface. After adding the stop valve, the air pump can be placed above the water surface or below the water surface, and there is no danger of water returning when the air pump is powered off and stopped working. 4.Lightweight, easy to install and use. 5.Wonderful accessory for aquarium, fish tank. Specifications: Material: plastic Color: black Length: Approx. 5.5cm/2.2in Suitable for air inlet / outlet Gauge Tube of 4mm. Packing List: 20 x Water Check Valve Note: Each outlet needs to be installed one. The double-headed air pump needs two stop valves to prevent the water in the hose of the aeration pump from flowing back to the machine and damage the front-end air pump or other product equipment.