Brian, look up the difference between the "weight" and "mass". I think you are talking about mass, not weight. Weight changes depending on what gas/fluid as object is suspended in, while mass is constant.

"The weight of an object in static equilibrium equals the magnitude of the gravitational force acting on the object, less the effect of its buoyancy in any fluid in which it might be immersed"

"everyday usage, mass is often taken to mean weight, but in scientific use, they refer to different properties."

"Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied."