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.
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 temperatures increase, for example, levels of dissolved oxygen (DO) decrease.
As salinity increases, DO decreases, and as atmospheric pressure decreases, so does the DO level. Aquatic plants and green algae also influence the DO in water. During the day, photosynthesis occurs and aquatic plants produce oxygen as a result. 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.
How to Avoid Hypoxia
To avoid hypoxia, make sure your fish are not crowded and have a functioning filtration/aeration system. Also, make a plan in case of unexpected power failure.
If the power goes off, don’t feed your fish: Eating only increases the demand for oxygen. Stir the aquarium water every hour or two with a whisk or other utensil to help increase oxygen levels. A battery-operated aerator, available in many fishing tackle shops, is a good piece of equipment to have on hand as a backup. You should also keep extra plastic bags and buckets around in case you have to move your fish during a breakdown.
You can monitor oxygen levels by using a water test kit. The standard test is somewhat time-consuming (10 to 15 minutes) but results are reliable. Electronic oxygen reading meters are also available but tend to be expensive.
By performing specialized water tests, your veterinarian can rule out disease problems similar to hypoxia including chlorine toxicity, gill disease and hypothermia. 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 her/his anesthesia machine into your fish’s 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 24 hours or more.