Why is there more lack of oxygen in hot water in the sky?

During the day, algae in the pond can carry out photosynthesis to produce oxygen, which greatly increases the oxygen content in the pond water. At this time, the fish can get enough oxygen from the water without floating their heads. Because of the lack of sunlight at night, algae in the pond can't photosynthesize to produce oxygen, and they need to breathe and consume a lot of oxygen. The oxygen content in the pond water is the lowest at dawn. At this time, in order to breathe, the fish will float their heads and even jump out of the water to absorb oxygen from the air; Therefore, the phenomenon of fish floating head is due to lack of oxygen in the water, which will lead to a large number of fish deaths in severe cases.