The fish and the plant take part in the carbon dioxide and oxygen cycle. The carbon dioxide and oxygen cycle is the process of trading or exchanging the gases oxygen and carbon dioxide between producers and consumers. Fish are the consumers, and plants are the producers. If gases were used up instead of exchanged in this cycle, living things would die. Gases are recycled in the carbon dioxide and oxygen cycle.
Animals breathe in oxygen and breathe out carbon dioxide.
Plants take in carbon dioxide and give off oxygen. Plants need some oxygen, too. During the day, plants make oxygen. At night, plants don't make food or oxygen. They get oxygen from the air, just as animals do.