How plants give off oxygen!
How do plants make oxygen?
Plants make oxygen when photosynthesis occurs when the plants makes food. The things you need for photosynthesis are water , sunlight ,and carbon dioxide. The plants roots suck in the water ,sunlight, and carbon dioxide after that the plant lets a gas out which is oxygen.
In this case the fish gives off CO2 (carbon dioxide) and the plant absorbs the CO2 and then the plant gives off the oxygen.