Are you diving an Aluminum 80 cylinder? These are known to be positively buoyant as the air in them is depleted.
Okay, let's settle this common misconception.Maybe I read your message in a different way than intended. But the material of the tank doesn't change the buoyancy shift between the beginning and the end of the dive.
Your buoyancy depends upon all the factors of your dive--your body, your gear, the air in your BCD, the air you breathe--everything. Some things pull you up toward the surface, and some things pull you down. It is not dependent upon one thing alone.
If you and all your gear are neutrally buoyant at the beginning of your dive, that is because all the factors of your dive--your body, your gear, the air in your BCD, the air you breathe--weigh the same as the water you are displacing.
If you lose 6 pounds of air during the dive and nothing else changes, you will be 6 pounds more buoyant at the end. If you want to stay neutral, you will need to lose the appropriate amount of air in the BCD to make up for the lost air.
It does not matter whether your tank is steel or aluminum--if it loses 6 pounds of air during the dive, it will be six pounds more buoyant than it was at the beginning of the dive.