How Do Fish Increase Oxygen Uptake From Their Environment?

Keep in mind that there are neither blaring alarms nor flashing lights when fish in open bodies of water and tanks experience low oxygen levels. That’s where the problem starts, especially with inexperienced aquarium owners – the fish may be dying and the interventions may be too late.

The importance of knowing the signs and being observant about them cannot be overemphasized in the interest of prompt and proper interventions. 

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Hypoxia, the scientific term for low oxygen level in the water, causes the fish to act in abnormal ways. While every fish species has its own ways of increasing oxygen uptake, they exhibit common behavior as discussed below.

Parents Protecting Their Young

Look for abnormal behavior in parent fish during the breeding season. Parent fish can exhibit increased fanning behavior, as characterized by increased frequency of swimming over or near the nest where the eggs are located.

The action creates a flow of water around the eggs and, thus, provides them with a constant oxygen supply.

Gobies, sticklebacks, and clownfish are known to exhibit fanning behavior during hypoxia. Also, gobies have been documented to increase the size of their nests’ openings at the risk of exposing their eggs to predators.

Again, the purpose is to increase oxygen uptake for their offspring.

Parent rainbow cichlids also have their way of coping with hypoxia. They often move their fry from the bottom to near-surface level since it’s where oxygen is more abundant.

If you observe parent fish in your tank showing this abnormal behavior, then you should look into the possibility of hypoxia.

Fish Decrease Their Activity

When oxygen levels in the water initially decrease, fish are more likely to move from the hypoxic zone to areas with more oxygen. They will seem like their activities are increasing with more swimming to and fro, rising to the surface and fanning the water, among others.

They may even seem so cute while doing so, especially when they appeared lethargic before.

But without appropriate changes in the tank’s environment, the oxygen levels will continue to drop and the fish will change its behavior again. They will move around less, swim less vigorously, and even eat less frequently in an effort to reduce their energy demands.

By doing so, they are also decreasing their oxygen demand, perhaps conserving the oxygen in the water. If you’re observant about your fish’s behavior, you will observe them seemingly becoming more lethargic.

Fish Engage in More Aquatic Surface Respiration

Eventually, fish will swim closer to the surface of the water, known as aquatic surface respiration (ASR). The surface layer of the water contains higher amounts of dissolved oxygen and, thus, fish are drawn to it.

The oxygen in the air diffuses into the water and since the top water column has greater surface contact with air, it has higher oxygen levels. 

But this only applies to stagnant water, such as in aquariums and tide pools. In open bodies of water, the running water means that the oxygen levels in the entire water column are the same. Still, fish will likely come to the surface in search of more oxygen.

But it should also be noted that ASR behavior by fish in hypoxic environments isn’t the same as surface feeding and breathing. Certain fish species, such as labyrinth fish, typically feed at the surface so their swimming to the surface shouldn’t be immediately taken as hypoxia.

Other fish species, such as gouramis and bettas, also take periodic gulps of air from the surface, and it’s considered normal behavior.

What then should you look out for, especially in aquarium fish? Fish striving to increase its oxygen uptake will go to the surface of the water, make repeated grasping movements for several seconds, and usually do so with a wide-open mouth.

Think of the actions of a drowning person swimming to the surface for air and you will get the picture.

Obviously, these ASR-related behaviors are intended to increase the chances of survival for the fish concerned. Certain fish, such as gestating sailfin molly, will perform ASR more often than a non-gestating female due to the higher demands for oxygen.

Fish May Also Engage in Aerial Respiration

Also known as air-breathing, aerial respiration in fish is characterized by their apparent gulping of air at the water’s surface. This movement is intended to extract oxygen directly from the atmosphere instead of waiting for it, so to speak, to be dissolved in the water.

In both ASR and aerial respiration, the risks of predation increases since these coping methods expose the fish to predators like birds and other fish-eating predators. But in aquariums, the risk is little to none unless you have a fish-loving cat.

Emergency Steps to Address Hypoxia

If you see your aquarium fish moving around less or swimming less, even gasping for air at the top, you should take swift action! This is true whether there’s only one bottom-dwelling fish doing so or the entire fish collection doing it.

The fish that aren’t showing signs of hypoxia likely need less oxygen for one reason or another but eventually they, too, will become victim to hypoxia.

  • Make about a 50% water change.
  • Increase water movement by putting an additional filter, air stones, or a powerhead, which will introduce more oxygen.

With these emergency measures done, you can then look at the possible underlying cause for hypoxia. The aquarium may have excess waste so adding a more powerful filter, such as the Tetra Whisper Power Filter for Aquariums, will remove it.

This filter works in three ways, namely, mechanical filtration with its dual-sided mesh, chemical filtration via ultra-activated carbon, and biological filtration via bio-foam.

The water temperature may also be too warm so the fish eventually feels the heat and oxygen deprivation. You should use the Zacro LCD Digital Aquarium Thermometer to monitor the aquarium’s water temperature. Just submerge its probe into the water and let it do its work.

There may be too many live plants and too little light in the tank, too. You should consider removing many of the live plants and installing the NICREW ClassicLED Aquarium Light over it. The energy-efficient, super-bright LED lights will not only aid in addressing hypoxia but also improve its aesthetics.


Like all living organisms, fish need oxygen to live and thrive. As an aquarium owner, you should be observant about your fish’s behavior in relation to possible oxygen deprivation.