When choosing a scuba cylinder, the first thing is to consider is the size and capacity of the tank based on factors such as body size, diving style, buoyancy, weight and air consumption. Generally, it is better to choose a larger capacity cylinder unless the person using it is quite small or needs to carry the tank long distances over land.
A good general guideline to use when selecting the size of the tank is that users should be able to sit down without the tank pushing the buoyancy compensator over their heads. Another good rule is to choose a tank that is short enough for users to carry it by the neck with their arms fully extended without needing to drag the tank across the ground to prevent damage to the tank.
After determining the appropriate size and capacity of the scuba cylinder, the next step is to choose between a steel or an aluminum tank. Steel cylinders are generally stronger and more resistant to damage, but they are also more prone to internal rust. Aluminum cylinders are more prone to external damage, but they resist internal corrosion better than steel cylinders. Steel cylinders are negatively buoyant when empty, which means that divers must remove weight from the weight belt in order to stay at the same depth, while aluminum cylinders are positively buoyant when empty, which means that divers must add more weight.