I have to add my 2 cents! The size as mentioned is not as important as capacity at a given air pressure. HP (High Pressure) tanks have a problem, that is filling them full! When you fill a tank it heats up and the air expands and then when it cools or hits the water it will drop about 10% to 20% of the volume of gas and the proportional pressure. A slow fill works better not heating as much. Now over 3000psi is hard to pump! It is hard on the equipment and uses the most energy and creates the greatest wear on the equipment. Most dive boats can pump 3000psi which then being a hot fill will be 2600 to 2800psi. So if you have a 80qft tank filled to 90% you have 90% of 80qft of air or 72qft of air. The steel tank can take the place of lead on your hip, but the aluminum tank will float when near empty. So if you have a 119qft steel tank rated at 3500psi and you get a fill that cools to 3000psa you have apx 100qft of usable air. The best is a steel 95 to 105qft Low pressure tank. Rated to hold 95qft at 2400psi. Now most boats and shops will fill it hot to 3000psi plus or minus and you have 20% over fill or 95qft will have close to 118qft of air compared to the HP 119qft that only has Apx. 100qft! So to me a LP steel is the way to go! So as you should have covered in your basic class the letters and numbers on the tank do matter. After you have been diving awhile as a woman you will blow the doors off most male divers as far as consumption!