Dissolved oxygen refers to the amount of oxygen dissolved in seawater, which is one of the important conditions for the survival of aquatic organisms in the ocean. Fish breathe through the contact of their gills with dissolved oxygen in the water. Molluscs breathe in other ways. Compared with the atmosphere, the dissolved oxygen in water is extremely small. In terms of absolute content, one liter of water only contains 5mg-7mg, which is 1/30 of the oxygen content in the atmosphere. And the oxygen content in the water has a great relationship with the water temperature.

When the seawater in the aquarium is hypoxic, what we can generally see is the breathing frequency of the fish in the water. That is to say, the number of incitements of the gills increases, and then rises to sleep and opens the mouth. In severe cases, fish and other creatures will die.