If you are doing deeper dives, or dives in an extended overhead environment, then you need that added time that a larger cylinder will give. The Zeagle 20 cu ft Pony Bottle System is a Totally Redundant Breathing System that includes a 20 cu ft Aluminum Cylinder, a Zeagle RaZor Regulator/Valve Combination with a 36' hose and a balanced Envoy 2nd stage. The system weighs roughly 12lbs (empty).
Note: There are MANY different ways to mount the 20 cu ft Pony Bottle System Redundant System to your BC. Every way of mounting extra tanks (valve-up, valve-down, valve-left, valve-right, etc.) has supporters and detractors. Be assured that you can mount it in your preferred fashion as well.
One way is to install a pair of 8029HD Pony Tank Bands in the back.
If the diver prefers to carry the 750-6220 Redundant System in front, then the 8079 Tank Pouch can be used.
The 8079 Tank Pouch is a VERY versatile way of mounting the 750-6220 Redundant Air System.