Water Quality: Dissolved Oxygen
Oxygen is as critical to fish life as it is to human life. DO refers to the oxygen gas that is dissolved in water. Fish absorb the DO directly from the water into their bloodstream through their gills. Although some species, such as bettas and gouramis, are capable of breathing air, sufficient oxygen levels should always be maintained in the water for all fish.
The amount of oxygen water can hold (saturation) is dependent upon water temperature, salinity and atmospheric pressure. As water temperature increases, the amount of oxygen the water can hold decreases. Likewise, as salinity increases, less oxygen can be dissolved into the water. At greater pressures, water can hold more oxygen. For example, more oxygen can be held in water (of equal temperature and salinity) in Miami than at the much higher altitudes of Denver.
DO may be supplemented by use of an aeration system. In an aquarium, this aeration system often consists of only a simple air pump, air tubing and airstone. While this type of system does not greatly increase the oxygen level by pumping air (and, therefore, oxygen) into the tank, it can help circulate the water and break the surface tension of the water to increase the rate of diffusion. Very small air bubbles, like those from a fine pore stone, that travel slowly from the bottom to the top of the tank are much more efficient in adding oxygen to the water than large air bubbles which boil the water or airstones which sit near the water surface.
Another excellent source of oxygen in an aquarium, and in many natural bodies of water, is plants. Plants produce oxygen as a by-product of photosynthesis, a process by which plants use light energy to produce food from carbon dioxide and water. In the presence of light, the plants consume carbon dioxide and produce oxygen during photosynthesis. In the absence of light, the fish, plants and other organisms in an aquarium continue consuming oxygen and producing carbon dioxide, but no oxygen is produced. Therefore, oxygen levels are lowest early in the morning after a night of respiration and no photosynthesis. DO is highest in the late afternoon after a full day of oxygen-producing photosynthesis. See Figure 1.
Aquarists should attempt to keep oxygen as close to saturation as possible. Without a meter to check the oxygen level in a tank, the best way to help ensure saturation is through good management. Add live plants to the aquarium and use an aeration system. Do not overstock the aquarium with fish and do not overfeed. Siphon out detritus (decaying material) from the bottom of the tank and clean mechanical filters regularly. See Aquarium Setup, Care & Maintenance. Be aware of chemical or medication treatment effects on the biofilter or directly on DO. A dissolved oxygen meter is a good investment for wholesale and retail operations and should be used to check oxygen levels each morning, after any chemical / medical treatments or if fish are seen gasping at the surface of the water.