Key Features High Oxygen Air Cylinder Made of silica for the finest air bubbles For use within Ponds or Aquariums Can also be used in ponds with the relevant airline. pack of 12 65mm x 15mm High-oxygen air cylinders with protruding end caps to keep them clear of pond liners. Made of silica for finest air bubbles and long life. The 65 x 15mm air cylinder is designed for use within aquariums, but can be used in pond set ups with the relevent airline tubing and air pump set up. This is a Pack of 12