How do aquatic plants and fish exchange oxygen and carbon dioxide in a balanced aquarium?

In a balanced aquarium, fish and aquatic plants maintain a reciprocal relationship regarding gas exchange.

Fish breathe by using their gills to absorb dissolved oxygen from the water and release carbon dioxide as a metabolic byproduct. Conversely, aquatic plants consume carbon dioxide and release oxygen during the process of photosynthesis, provided they receive sufficient light.123

This cycle creates a balance where the animals provide what the plants require for photosynthesis, and the plants provide the oxygen necessary for the animals' respiration. Additionally, the waste products produced by fish can serve as fertilizer or food for the plants. While this system is theoretically self-sustaining, achieving a true balance is difficult in practice, as it requires precise light intensity and careful stocking levels to ensure the plants and animals can effectively offset each other's respiratory needs.34