Why can you dive with a small tank without a BCD.

When you dive with a big tank, at the end of the dive you have a big empty tank that is buoyant (for example an empty aluminum 80 cubic foot tank has over 4.1 lbs. of buoyancy). To compensate for this a diver has to add 4 extra pounds on top of the weight necessary to overcome the buoyancy of the body. Its hard to tread water with 8 – 10 lbs. of weights, so a BCD is very useful as it can really reduce the work of swimming with weights. A 24 cubic foot tank is neutrally buoyant when empty, thus the only weight is necessary is to compensate for the human body buoyancy. When carrying only 2 lbs. of weights it is practical to swim on the surface without the aid of an inflated BCD.