Buffalo Blizzard water bags are the best way to keep your pool cover secure all winter long. The water bags are manufactured from quality heavy gauge vinyl treated with UV inhibitors and anti-bacterial and anti-fungus additives. They are designed to withstand sub-zero temperatures and it is optional if you choose to use them with the loops on your winter cover. Instructions: Fill water bags 1/2 to 3/4 full to allow room for expansion due to freezing. If you are having trouble closing the valves on your water bags - try this trick! Soak them in warm water and they will close easily.