Dnadavis and rezgrama both had good answers. And it does take time. You will typically always use more in the beginning of the season too. For me the key is to be comfortable not getting exactly where I may want to go immediately. For example, If you want to sink in a hurry, just by exhaling, then that extra weight makes it easier. A steel tank will help, as they aren’t typically as positively buoyant, and buoyancy isn’t as drastic as aluminum between full and empty.