Divers can choose between scuba tanks manufactured from aluminum or steel. Aluminum tanks are lighter than those, made of steel and, thus easier to transport and swim with. They are also considered to be more cost-efficient. One thing you need to bear in mind when it comes to aluminum tanks is that because of their lightweight, some aluminum tanks will begin to float at the end of a dive, as you use up your oxygen supply. In order to cope with this and counter-weight the tank, divers usually carry a few additional pounds of weight.
Steel tanks are stronger and more ductile than aluminum ones. If properly cared-for steel scuba cylinders can last for decades. An important benefit of using steel tanks is better buoyancy characteristics. The downside of using steel tanks is that rust can be a factor over time and maintenance is always going to be extremely important.
As for the dive tank pressures they can vary a great deal. There are three pressure ranges - "low" (2400 to 2640 psi), "standard" (3000 psi), and "high" (3300 to 3500 psi). The most common service pressures seen in modern scuba tanks are 2640 psi, 3000 psi, 3300 psi, 3442 psi, and 3500 psi.
Tank coatings fall into three general classifications of painted, galvanized, and uncoated. The aluminum tank finishes of choice are unpainted, brushed, shot blast, and natural. Uncoated steel tanks quickly rust in the presence of water, so all steel tanks are coated with zinc.