What gas is produced by fish and used by aquatic plants?

The gas produced by fish that is utilized by aquatic plants is carbon dioxide (CO2). Fish excrete carbon dioxide as a byproduct of respiration, which dissolves in the water and provides a vital source of carbon for aquatic plants during photosynthesis. In turn, these plants release oxygen, which is essential for fish and other aquatic organisms. This cyclical relationship helps maintain a balanced ecosystem in aquatic environments.