People who keep goldfish often put a few water plants in the fish tank. Don’t think that the water plants are merely decorations; they have other uses!
When we are in a closed room with no ventilation, the air becomes stale and low in oxygen, making us feel stuffy. However, once we step into the embrace of nature and breathe in fresh air full of oxygen, we immediately feel much better.
The same situation applies to fish. In summer, fish in ponds often come to the surface to breathe fresh air.
In a goldfish tank, placing a few water plants serves the purpose of establishing some “oxygen factories” for the fish. Water plants utilize the carbon dioxide dissolved in the water, mix it with the abundant water around them, and use the energy “captured” from sunlight to produce what they need. During this process, they produce a byproduct—oxygen—which they generously provide to the fish, allowing the goldfish to breathe freely.