Aluminum tanks are cost efficient, long lasting, and readily available, which is why they are easily the most common scuba diving tanks in the world. Lighter than steel tanks, aluminum dive tanks are easier to transport and swim with, giving your shoulders and arms a break on long treks.
Because of their light weight, some aluminum tanks will begin to float at the end of a dive, once the oxygen supply is nearly depleted. To combat this, it's common to counter-weight an aluminum dive tank with approximately 4 pounds. However, 'Compact Neutral' aluminum tanks are made with thicker walls, which increase the weight and help to counteract the positive buoyancy problems at the end of a long dive.