Abstract: An anti-siphon fluid tank adaptor to prevent the insertion of a siphoning tube while nonetheless permitting rapid filling of the tank.The adaptor comprises an extension firmly secured to the tank and a tubular sleeve firmly welded to the extension and having an end portion within the tank covered with a conically-shaped member. The sleeve is provided with several holes about the circumference of the sleeve and one end of the extension is provided with means for receiving a cap for closing the entire adaptor.