Can Fish Drown? – 4 Surprising Causes

Have you ever wondered: Can fish drown? Kids and adults around the world will be quick to conclude that fishes cannot drown. But, what is the truth? Fishes spend all their life in water so drowning may seem impossible.

Surprisingly, fishes can suffocate and drown in water too! Yes, many factors in the water can affect the quality of life and longevity of a fish. If you are in disbelief, you are not alone!

In this article, you can get a detailed overview of how fish can breathe underwater and what factors can cause them to drown.

Plus, if you have pet fishes at home you can also learn how to improve the tank’s oxygen level. Continue reading to explore this interesting topic.

Do Fish Drown Without Oxygen?

Fishes depend on oxygen the same way we do. Instead of breathing in oxygen from the air, fishes live on dissolved oxygen present in the water they live in.

So, do fish drown without oxygen? Yes! Without adequate amounts of oxygen, fishes cannot respire, and will eventually die and drown.

Oxygen combines with other elements to form protein. This helps fishes form new cells. Furthermore, oxygen is essential for proper metabolism.

It helps break down food, and convert and store its energy. This energy is essential for carrying out active processes and bodily functions.

Fishes in the aquarium often struggle because there is very little water and the tanks can run out of oxygen very quickly.

This is why aquarists must keep the tank size in consideration. Adding some plants to the tank may also be a good idea as plants photosynthesize and release oxygen.  Without these or artificial air pumps, pet fishes will die.

Inner Organs of a Fish to Aid Underwater Breathing

Of course, breathing in water is not as easy as taking in the fresh air on land. Thanks to the cold-blooded nature of fishes, they do not require much oxygen.

Only a finite amount of oxygen is present in a water habitat. A larger surface area of the body of water can result in more oxygen. To cope with the limited supply of oxygen underwater, fishes have specialized internal organs.

Fishes rely on special organs called gills to extract oxygen from the water. Gills are feathery organs composed of blood vessels.

Most of them have a set of gills on both sides of their body. Gills can be found under the operculum, also known as the gill slit. While many fishes have four pairs, sharks can have up to seven pairs of gills.

Fishes have other specialized features to cope with factors like salt level. This is helpful as it can affect how much oxygen is readily available in the water.

Fishes who live in saltwater need a specialized mechanism to get rid of excess salt in their body. On the other hand, freshwater fishes try to hold as much salt as possible with their evolved gills.

Do Fish Have Lungs?

It is not normal for fishes to have air-breathing lungs like mammals. To function properly, lungs must be empty of fluids.

If fishes had lungs, the tiny air sacs in their lungs would fill up with water when trying to pull oxygen. You will be surprised to know that a handful of fishes have lungs. 

Popularly grouped as the Lungfishes, they are six freshwater fishes found in South America, Australia, and Africa. These species can survive even if the water in their habitat dries out.

They burrow into the substrate, slow their metabolism, and enter a temporary state of hibernation.

How Do Fish Breathe?

Fishes breathe by taking water in through their mouth and then forcing it out through the gills. As the water passes the gill passage and thin walls of the organ, the gills extract the dissolved oxygen.

This dissolved oxygen is then distributed to the fish cells through blood.

Areas in the body where there is too much carbon dioxide receive oxygen when the fish respires. Carbon dioxide is then carried out through the gills. This exchange of oxygen and carbon dioxide is the same as human respiration.

Isn’t it interesting how we have the same anatomical functions as fishes even though we are so different?

How do Fish Suffocate?

It is worth mentioning that fishes cannot take in oxygen when it is chemically attached to hydrogen – H2O as we know it.

Fishes can only use dissolved oxygen, O2, present in the water body. If the water lacks a sufficient amount of dissolved oxygen, the fishes in it can suffocate. But can a fish drown from suffocation? Yes, absolutely.

The condition where fishes suffocate due to lack of oxygen is called asphyxiation. Sometimes, damaged gills can lead to this condition.

Environmental factors may also lead to lower dissolved oxygen concentrations in the water. Some of these factors are increased temperature, water pollution, overcrowding, excessive algae growth in the water, etc.

Can a Fish Drown? Factors That Cause Fish Drowning

Let’s explore some of those factors that can lead to fish drowning.

Eutrophication and Algae Blooms

Eutrophication is a condition when a high amount of nutrients such as phosphate and nitrate from fertilizers pollute the water. This results in algae bloom, which is when algae grow faster than the ecosystem can handle.

When algal blooms die, the microbial decomposition process uses a significant amount of oxygen, thus depleting dissolved oxygen reserves in the water.

Water Condition

If the water contains high concentrations of salt and other dissolved minerals, it will have low oxygen concentration. This is why freshwater bodies have more dissolved oxygen than saltwater marine habitats.

Water temperature is another factor. Cold water holds more dissolved oxygen compared to warm water. So, maintaining optimum water temperature can ensure adequate oxygen supply for fishes.

Ammonia and nitrate in the water can enter the fish and bind to the hemoglobin which originally attaches with oxygen and transports it.

If this happens, the fish will be unable to transport enough oxygen to the organs and therefore, suffocate painfully and drown.

Diseases and Parasites

What happens when gills are damaged due to various diseases and parasite invasion? They cannot breathe!

Unfortunately, both external and internal parasites are very commonly found on the gills. Flatworms from the Monogenenea class are among the commonly found parasites.

The gill fluke, another slimy parasite, is a deadly culprit for suffocation among carp, koi, goldfishes, and other fishes from the Cyprinidae family. Flukes lead to inflamed gills.

The irritation instigates the host to scratch the gill on hard surfaces and damages it. Ultimately, the fish will feel breathing difficulties, lose appetite, and become lethargic.

Parasite attacks are more common than you would think. For, these highly infectious species are able to multiply quickly. Aquarium fishes are exempted from the pestering of parasites.

Amongst other reasons, Ichthyophthirius multifiliis on gills of aquarium fishes can cause fishes to drown.

What’s more, fishes can drown due to fungal or bacterial diseases such as bacterial gill disease and amoebic gill disease.

If the bacteria or fungus overpopulates, the fish will not receive enough oxygen, and drown as a result.

Body Modifications

Often times, beautiful aquarium fishes are bred together to result in beautiful versions.

While this selective breeding is great for the aquarium trade, it can cause bodily modifications that are not ideal for swimming or oxygen absorption. The betta fish is a good example. 

Betta fish have a special respiratory organ called the labyrinth organ. While betta fishes breathe underwater via gills, the labyrinth organ allows this species to take in atmospheric air from the water surface.

Some long-tailed varieties of this fish can even gulp air from the surface and pull water in through gills.

However, overbreeding of this variety causes a body modification where their tail pulls them down when swimming. This exhausts the fish so much that it often leans on the bottom of the aquarium or other decorative items.

Finally, if the water lacks good oxygen flow, betta fishes will drown.

How to Identify Low Oxygen in the Water?

Surely, there are professional meters for labs and fields to test the oxygen levels in the water. You can also get an inexpensive home kit to conduct a DIY test for dissolved oxygen, pH, temperature, and turbidity.

However, one of the easiest ways to identify low oxygen levels in water is to notice how the fishes are behaving.

When there is less oxygen in water, fishes express signs of fatigue by moving less. In addition to swimming less vigorously, they also eat less often.

If fishes are floating to the surface to breathe, be informed that they will have a slow and nasty death if the oxygen in water does not increase.

This is why we recommend changing the aquarium water every week and running an airstone.

How to Improve Aquarium Oxygen Levels?

Thankfully, you can take some measures to improve your aquarium’s dissolved oxygen level. Below are a few suggestions to help ensure a good amount of oxygen is available in your tank for the fishes living in it. 

Number of Fishes

First and foremost, you will have to make certain that the aquarium is not overcrowded. As the number of fishes in a tank increases, the amount of oxygen supply for each fish reduces.

So, how many fish is too many?

Well, a general rule is to stock an aquarium following the one-inch per gallon rule. This means you can keep one inch of fish for every gallon of water in the tank.

As fishes vary in shape, size, and activeness, you might have to adjust this rule depending on what kind of fishes you have.

Stocking a 10-gallon tank with 10 inches of full-bodied goldfish is definitely not the same as stocking it with 10 inches of slender Zebra Danios.

Naturally, larger fishes have more waste and need more water. Plus, they also need room to swim.

You will require more space for schooling fish as they like to be kept in multiples. Having only one or two schooling fish will cause stress and reduce their lifespan.

Don’t forget to consider that some fishes grow significantly over time. So, do your research to determine whether your aquarium is big enough to accommodate the fully grown fish.

Water Surface Area of the Aquarium

By now you know that the larger the water surface area, the more oxygen exchange will take place. This, in turn, supports more fish.

So, you can keep more fish in an aquarium with more surface area. A short and wide tank may hold the same volume of water as a tall and thin tank, but the oxygen exchange will vary significantly due to the surface area.

Water movement

The oxygen level in stagnant water will always be low. So a great way to increase the oxygen level in water is to create movement in the water.

Find ways to circulate water so that the oxygen in the tank water surface can spread throughout the water.

One of the best ways to do this is to use filters. Filters create movement at the surface and boost oxygen exchange. Basically, filters take water from the bottom of the aquarium and bring it to the surface.

Therefore, creating an even distribution of oxygen throughout the tank. If your existing filter is not offering the desired result, you can replace it with a higher capacity filter.

You may also opt for an airstone, a powerhead, or a spray bar for the filter.

Oxygenation can be improved in ponds by adding a fountain. Fountains do a great job at aerating the water but anything that can move the surface water will be helpful.

Aquarium Temperature

As already mentioned, water can hold more oxygen when it is at a colder temperature. You can change the water in your aquarium with colder temperature water to introduce fresh oxygen to the tank.

Even a partial water change in the 65 to 70 degree Fahrenheit range will lower the temperature.

If you think the aquarium is overheating, turn off the lights and heaters. Moreover, you can remove the aquarium cover and blow air from a fan to cool the water.

Be sure to keep the normal temperature range for the fishes when adjusting the temperature.


So the answer to your question, ‘do fish drown’ is a yes. Even though it is their nature to swim in the water, drowning is a possibility. However, drowning of healthy fishes can usually be avoided if proper oxygen supply in water is ensured.

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