The bubbles will descend, and then hover on the denser layer of carbon dioxide gas. After a few minutes, notice that the bubbles begin to expand and sink. Notice the color bands on the bubbles. Watch how some of the bubbles freeze on the dry ice. As dry ice turns from solid to vapor, or sublimes, it produces carbon dioxide gas.
Carbon dioxide is denser than air.
The denser carbon dioxide gas forms a layer on the bottom of the aquarium. A bubble is full of air. It floats on the carbon dioxide layer just like a helium balloon floating in the air. You might expect that the air in the bubble would cool and contract near the dry ice, but the bubble actually expands slightly. The soapy wall of the bubble allows carbon dioxide to pass through, but does not allow air molecules to pass through.