O2 Mini-Lab Test Kit

The gas Oxygen (O2) comprises of approximately 21% of the volume of air and is vital for the existence of life on earth. Together with carbon dioxide (CO2), oxygen forms the closed cycle of gas exchange between plants and animals.

Through contact with the air and the photosynthetic activity of green water plants, oxygen dissolves in water. The maximum amount that can dissolve (the saturation level) depends on the temperature and salinity of the water. The warmer the water and the higher the salinity, the lower the saturation level. Cold freshwater is therefore considerably richer in oxygen than warm seawater. The saturation levels for several temperatures and salinities are given in the following table.

If the level of dissolved oxygen decreases below a certain critical level, fishes and other aquatic animals will suffer from oxygen deficiency, eventually leading to suffocation. Since aquatic animals require more oxygen at high temperature and at the same time less oxygen is able to dissolve, most problems occur at unsuitably high temperatures. Oxygen deficiency can also be caused by over population of the aquarium, rotting organic material in the tank, a fouled filter or insufficient water circulation.

In case of oxygen deficiency fishes and other aquatic animals are usually seen breathing fast or even gasping near the water surface, where the oxygen concentration is slightly higher.

Although plants and algae produce oxygen by day, they consume oxygen at night, so that extremely low oxygen levels may occur during the night. This usually happens in tanks with a lot of algae and insufficient water circulation.