Thunderbird Cylinders 30 Cubic Feet Aluminum Tank with Standard Thermo K-ValveThe Thunderbird Aluminum SCUBA tanks are durable, reliable, and visually appealing. The tank is manufactured from the industry standard lightweight, high strength aluminum 6061 T6 alloy. The 30 cubic foot tank is an ideal volume cylinder for use as a large capacity pony bottle, redundant air supply system, bailout or moderate duration decompression safety stop gas supply. A pony bottle is a smaller tank, which is carried in addition to the main tank.