Scuba tanks or cylinder is what you will be using to carry your air around. They come in all shapes and sizes and are made from steel or aluminium. Probably the most common size is 12 litres although you can purchase less or more depending on your preference.
What you choose will invariably depend on the conditions and depth that you will be diving. I started out with a 12 litre and for me I found this was too small. In my local area, a lot of divers use doubles and when I was buddied up with them I couldn’t keep up. So I purchased a 15 litre and sling a pony bottle. With this set-up, I can do 40-metre dives and have plenty of air in reserve. You can also choose between low-pressure and high-pressure tanks, a high-pressure tank as suggested will hold more compressed air.
As you may have guessed steel is stronger than aluminium, although due to the thinner walls they weigh less than a same size aluminium equivalent. One of the major benefits of steel is that they are less buoyant than aluminium, especially as the tank becomes empty. This means that towards the end of your dive you won’t have to compensate as much for the change of buoyancy. The problem with steal is that it can rust, although with proper care and maintenance this should not be an issue and you will see many years of use from it.
Tanks need to tested regularly to ensure they are structurally sound this is done by what is called a hydrostatic test. A hydrostatic test works by pressurising the cylinder with any changes in size measured. In Australia it is law that all tanks are tested annually and if your tank is out of date you will most likely be refused a fill. Additionally aluminium tanks are required to be tested ultrasonic for possible cracks. If you are from Melbourne I can highly recommend Scuba doctor for your servicing needs.