Hypoxia is the name of the problem in which there is too little dissolved oxygen in an aquarium or pond. The amount of oxygen dissolved in a given volume of water depends on four factors: temperature, atmospheric pressure, salinity and the number of aquatic plants in the system. As salinity increases, for example, DO decreases, and as atmospheric pressure decreases, so does the DO level; water at 40 degrees Celsius contains twice as much oxygen as water at 400 degrees Celsius. Generally speaking, cold and cool water fishes require more oxygen than warm water fishes.
Aquatic plants and green algae also influence the DO in water. During the day, photosynthesis occurs and aquatic plants produce oxygen. At night, these same plants absorb oxygen, and if numerous, they can deplete a body of water of available oxygen. This is rarely a problem in the home aquarium but is common in large shallow ponds during the hot summer months.
In the majority of hypoxia cases involving pet fish, a pump, fountain, or filter failure is usually at the heart of the problem. Contributing factors include increased environmental temperatures, over-crowding, and over-feeding. Too little oxygen in an aquarium's water can have deadly consequences. When a pump, fountain or filter fails, fish rise quickly to the water's surface, gasping for air. If they aren't treated quickly, they may die.
Most fish must ventilate a volume of water ten times that of the air ventilated by a terrestrial animal of similar size in order to obtain the same quantity of oxygen. In water, oxygen is either saturated (the water is holding all of the oxygen it can), supersaturated (the amount of oxygen exceeds the water's ability to hold it and there is too much oxygen in the water), or undersaturated (there is room in the water for more oxygen). With adequate aeration, most home aquariums are saturated and contain enough oxygen for the tank residents.
By performing specialized water tests, your veterinarian can rule out other disease problems that are similar to hypoxia including chlorine toxicity, gill disease and hyperthermia. If hypoxia is diagnosed by your veterinarian, the prognosis is usually good if the source of the problem can be resolved quickly.
Your veterinarian can also provide oxygen to your fish by bubbling oxygen from the anesthesia machine into your fish¹s water or sealing your fish in a plastic bag with one third water and two-thirds oxygen. This type of emergency treatment can keep your fish alive for up to twenty-four hours or more.
Most of the larger water test kits contain a test for oxygen levels. The standard test is somewhat time consuming (10-15 minutes) but results are reliable. Electronic oxygen reading meters are also available but tend to be expensive. Even if you don't have an oxygen test kit at home, it¹s a good idea to keep some extra plastic bags and buckets around in case you have to move your fish in the event of an aquarium emergency.
As with other water quality problems, prevention is the best solution. By insuring that your fish are not crowded and have a functioning filtration/aeration system, the chances that they will be exposed to hypoxia is relatively small.
One of the best things you can do to fend off hypoxia is have a plan for an unexpected power failure. Fish should not be fed during a power failure, as this will only increase their oxygen demand. Manually stirring the aquarium every hour or two with a whisk or other utensil can help put needed oxygen into the aquarium water. A battery-operated aerator, available in many fishing tackle shops, is a good piece of equipment to have on hand for a pump or filter failure emergency.