There is an art to breathing slowly and exhaling with relaxation. You use less air and can dive for longer using one tank. The guys behind me were using air so fast that we had to surface while I still had plenty of air. I didn’t understand until I asked the instructor. Happily my recent fitness and cardiovascular exercise program have made me efficient. It means that I can average about 55 minutes on a tank. Remember when Mike Nelson in ‘Sea Hunt’ swam around with two tanks on his back. Let me tell you that it’s way too heavy to carry two tanks. Really!