Answer:
oxygen bubbles
Explanation:
These bubbles are oxygen bubbles that are being produced by the aquatic plants at the bottom of the aquarium. As oxygen usually does it floats to the top of the water and back up into the atmosphere. This happens because of the photosynthesis process in which the plants soak up sunlight and carbon dioxide and uses it to produce and release oxygen. Since this process happens underwater for the aquatic plants the oxygen become bubbles and float up.
