It floats if you have balloon filled with helium. Likewise, a balloon filled with CO₂ will fall to the floor faster than a balloon filled with normal atmospheric air.

But we are not dealing with gas sequestered into balloons. The kinetic energy of the moving gas will cause them to diffuse and evenly distribute in a container under normal conditions in a period of time that is inversely proportional to the mass of the gas molecules.

The mushroom beds are generating additional CO₂, which is concentrated at the bottom of the terrarium. If the mushrooms were removed, even without any outside agitation, the gasses would quickly diffuse and mix homogenously. But our mushrooms at the bottom of the terrarium are little CO₂ generators. This gives the appearance of the "heavy" CO₂ sinking to the bottom, but this is not the case.

When you are dealing with a localized concentration of CO₂, you can pour it like a liquid, but this condition will not last long. The movement of the gas molecules is greater that the pull of gravity, so the gas will disperse. Pour some CO₂ into a container and see how long it stays there. I'll post some quick calculations to show how weak the force of gravity is compared to the kinetic energy of a gas at room temperature for all to see.

Meanwhile, ponder this example. Open a bottle of perfume or aftershave. You don't smell it immediately but eventually you will. There are two forces at work: the air currents in the room wafting the molecules and the diffusion of the gas. Aromatics, such as the molecules that make perfume and aftershave smell are much heavier than air, or carbon dioxide, but they will still diffuse through the air. Even in a "still" atmosphere, the heavier aromatics will reach your nose in a short period of time. The force of gravity is inconsequential compared to the energy at which they bump around in the air.