Can Fish Drown? Let’s Discover If This Is Possible

can fish drown

While watching your fish swim around their tank, you may look for signs of illness and watch for scale discoloration, odd swimming, or lack of appetite.

You may notice one day that your fish does not seem to be moving around the tank as much or is moving their gills more rapidly than before, but you do not see any physical problem.

What’s happening? Sometimes, fish may not get enough oxygen from the water around them, which leads to death if not corrected.

So, can fish drown?

Some species of fish can drown depending on the type of respiratory organ they possess. Fish with gills cannot drown, but they can suffocate. However, the few fish species that possess lungs could drown.

By definition, drowning is being unable to breathe or collect oxygen because one is submerged in liquid. Suffocation occurs when one is unable to breathe or is deprived of oxygen. 

While the difference between drowning and suffocation may seem insignificant, it is important to know as a fish owner.

Some people are unaware that their fish can die from lack of oxygen while in water and are therefore unprepared to prevent it. Today, we will explore how this can happen and how to make sure that your fish is getting enough oxygen.

How Fish Breathe

To understand why this unfortunate situation occurs, we must understand how fish collect oxygen from the surrounding water. Typically, fish use their gills, but there are a couple of other respiratory systems as well.


When most people think of fish, they think of gills. The gills are complex structures. They are composed of many filaments made up of even smaller structures called “lamellae.”

The lamellae and filaments are made of very thin tissue and have a lot of blood flow. This allows oxygen in the water to enter and carbon dioxide to leave the gills’ blood as water runs over these structures.

Labyrinth Organ

The labyrinth organ is found as an accessory breathing organ in fish with gills. This organ allows the fish to take gulps of air at the water’s surface and collect the oxygen as it passes through the labyrinth, situated above the gills.

Fish perform this kind of respiration when the surrounding water has low levels of oxygen. One of the most popular species of pet fish, the betta fish, has the labyrinth.

Swim Bladder or Lungs

Most fish have a swim bladder to help with buoyancy and maintaining appropriate depth in the water. However, species like lungfish and gars use their lungs or swim bladder to help them breathe in addition to (or instead of) their gills. 

Lungfish have a primitive lung structure that developed just before the swim bladder on the evolutionary timeline. Australian lungfish will use this when there is not enough oxygen in the water to use their gills effectively, while all of the other lungfish species always use their lungs.

They will go to the water’s surface and suck in air, and the blood vessels lining the walls of the organ will absorb the oxygen. While not widely observed today, there are a few other kinds of fish that rely solely on lungs and air to obtain their oxygen.

Gars have a swim bladder that can act as a lung in water with low oxygen. The water temperature also influences the frequency with which they use their swim bladder and “gulp” air. Higher temperatures prompt them to breathe more air versus rely on their gills.

Due to their respiratory flexibility, gars can live in various habitats, but most are primarily found in shallow freshwater. Like lungfish, gars are not the only kind of fish that possess a swim bladder that aids in respiration, but one of few.


Some fish have adapted to perform oxygen and carbon dioxide exchange through their skin in addition to gills or another respiratory organ. The proportion of oxygen obtained through the skin depends on the species of fish, habitat, and water temperature.

Causes of Suffocation

Now that we know how fish gather oxygen from their environment let’s dive into the causes of a lack of oxygen. Most of the fish kept as pets have gills or gills and a labyrinth as their primary respiratory structures, so that we will focus on these types of fish for now. We will touch on ones with lungs a little later.

In Water

While our atmosphere on Earth is about 220,000 parts per million oxygen, water only contains about 10 parts per million oxygen. Because of this, it is easy to deplete the level of oxygen by:

  • Overcrowding: If there are too many fish in one tank or aquarium, it may cause a lack of sufficient oxygen since they will all compete for a limited amount.
  • Increased Water Temperature: Cold water holds more oxygen than warm water. Suppose the water is too warm, too much oxygen escapes into the environment, and does not leave enough for your fish.
  • Waste: Fecal matter, food, and other waste limit the amount of oxygen that the water can absorb or dissolve from the surrounding air.
  • Algae and Plants: Too many algae in a dirty tank or too many plants will limit available oxygen as they use it to produce energy at night when no sunlight is available. The production and absorption of oxygen and carbon dioxide that algae and plants facilitate also affect the water’s pH in the tank.
  • Stagnant Water: If the water in the tank is not moving, the water at the surface becomes oxygenated but does not disperse throughout the tank.
  • Water Additives: Some water additives or conditioners lower the water’s carrying capacity for oxygen, so less oxygen will be absorbed from the air.

As you can see, many things could cause insufficient oxygen for your fish. Before we go over how to prevent and fix low oxygen, let’s discuss the signs that your fish may display if they are experiencing it.

Your fish will likely begin alerting you to the problem by not moving around their tank in the usual manner. They may slow down their swimming speed and eat less food than usual.

Eventually, they may begin moving their gills more quickly to filter more water and pick up more oxygen. They may go to the water’s surface and “pant” to draw air in and provide oxygen to their gills.

By the time you see low oxygen symptoms, immediate changes must be made to help your fish survive. 

Out of Water 

But what about taking a fish out of water? Since the air has a much higher concentration of oxygen, why can fish not survive?

The answer to this question lies in the physiology of the gills. They have maximized their surface area to encourage a high oxygen and carbon dioxide exchange with the surrounding water. The gills’ filaments are separate from one another, and water can pass between them to reach all of the lamellae. 

When a fish is taken out of the water, the gills “collapse” in a sense. The filaments are no longer suspended and able to remain apart like they do in the water. They begin to stick together, and the gills’ surface area decreases drastically, preventing enough oxygen from entering the blood circulating through the gills.

Fish That Utilize Lungs or Swim Bladders 

All but one species of lungfish rely on only their lungs for oxygen intake. If they can not air from the surface, they have no alternative to collect oxygen from the surrounding water and die.

This qualifies as drowning since the water prevents them from collecting oxygen and makes them a rare example of a fish that can drown.

For fish that use their lungs or swim bladders when the surrounding water’s oxygen content is low, like gars, it is harder to determine whether or not they experience drowning, but they do not.

If they cannot access air from the surface of the water, they would need to continue to use their gills and would not receive enough oxygen, which would result in suffocation.


The types of fish that are typically kept as pets in tanks and aquariums possess gills (or gills and labyrinths) and cannot drown. Unfortunately, they can suffocate due to low oxygen levels in the surrounding water.

Low oxygen can be caused by various factors and needs to be corrected as quickly as possible when noticed. Fish that utilize lungs or their swim bladders as secondary respiratory organs (with gills being primary) cannot drown.

They would also experience suffocation due to a lack of oxygen being absorbed by the gills. Fish that rely solely on their lungs and air from the surface for oxygen are the only type of fish that can drown.

John Kilmerstone

I love keeping pet fish and receive a lot of joy and peace from watching these colorful creatures. Please visit this website and explore the wonderful world of pet fish. Discover how to care for and look after pet fish and amplify your satisfaction.

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