Airpump Non Return Check Valves X 2 Pcs ? Pack of (2) Two Non Return Valves ? Will fit standard airline of 5/16 or 4mm ? Stops water entering airpumps Check valves or non return valves fit between the air pump and aeration device in the aquarium or pond. If you suffer a power outage and the pump is below the level of the aquarium like in the cabinet water will siphon back down and damage the airpump. These little devices stop that! Can be used in tropical aquariums or water garden koi ponds. We sell many aquatic products online, check out our store! Terms and Conditions. We use PayPal for taking payment, if you wish to purchase via other methods please email us for customer service. Sales Tax will be added to all shipments to New Jersey. We always try to combine purchases, please contact us directly for a personal service. You will be notified on the day that your goods have been dispatched via e-mail with tracking which is used on all purchases. Please note that if there are any problems, contact us first via e-mail giving full details of any problem along with your user ID. We use USPS and UPS as our chosen carriers shipments to Hawaii, Alaska Puerto Rico may incur surcharges, contact will be made to advise before shipment at which point the order can be canceled if desired. Orders placed on Saturday/Sunday Public Holidays will be processed next working day. We pride ourselves on excellent feedback, if any part of the buying experience is not to your satisfaction, please contact us to rectify before leaving feedback. By committing to buy you are agreeing to the Terms Conditions stated above.