Does the system per plants work like the filter of a conventional swimming pool?

Aquatic plants produce oxygen like terrestrial plants. Oxygen is soluble in water. The amount of oxygen within the water depends on the temperature thereof. In the afternoon, after a few hours of light, photosynthesis has produced so much oxygen that the limit of mixing water and oxygen will be exceeded. This means that oxygen bubbles start to come out of plants like carbon dioxide in a glass of sparkling water.

In an oxygen-filled aquatic environment there is no hypothesis that pathogenic organisms multiply in excess.