Will fishes drown if they stop swimming? Most fish lovers and owners would have wondered this at least once. Some would insist that their fish drowned in the aquarium. Besides, curiosity prompts many to think if fish can drown underwater. Hopefully, you have not lost a bet on this so far!

It is not an accident if you are reading this article. It critically examines fish drowning with a detailed explanation, which we should know, to prevent mishaps.

What is Drowning?

The dictionary meaning of drown (verb) is to “die from being submerged in water, getting water into the lungs, and asphyxiating.” It means death due to immersion in water and lack of oxygen due to water-filled lungs.

Fishes are born in water. They are already submerged in water and naturally swim and breathe underwater. Most do not have lungs. Hence, it is technically impossible for fish to drown in water.

They breathe the Dissolved Oxygen (DO) from water, just as we breathe oxygen from the air. They may suffocate to death in water for various reasons.

The word fish drown (now on) means they suffocate. It is used interchangeably for ease of understanding.

What Leads to Fish Suffocation

Two main causes of fish drowning are:

  • Dissolved Oxygen (DO) in water.
  • Other factors (that prevent fishes from inhaling DO).

DO in Water

Oxygen (O2) from the air dissolves in the water due to surface interaction caused by water currents, waves, ripples, flowing streams, underground water flow, smaller water bodies merging into larger water bodies, etc.

While the atmospheric air has 21% or 210000 oxygen Parts Per Million (ppm), DO in water is only 4 – 8 ppm. Being cold-blooded, fishes need less O2 to survive than us and can manage with this DO.

Lower DO levels make fish breathless. It may result in health complications due to increased stress, aggression, and poor water quality, proving fatal for them.

The DO level in water gets impacted by the following factors:

  • Still (non-moving)/slow-moving waters have lower DO levels than flowing water. That explains the importance of an air pump in a fish tank. The air bubbles move upwards and burst on the top, causing gaseous exchange, helping atmospheric O2 to dissolve in the tank water.
  • DO is highest on the top water level and diminishes with depth.
  • Aquatic plants and planktons are rich sources of DO in the water. In sunlight, they use carbon dioxide for photosynthesis and release O2, naturally oxygenating deep waters.
  • Warmer waters have lower DO than colder waters. Climate, seasons, and weather impact water levels and temperatures, fluctuating the DO levels.
  • Saline water has lower DO than freshwater, as some of its space is occupied by marine salt, reducing the free space for oxygen molecules to dissolve.
  • Water bodies with larger surface areas have higher DO than smaller ones, as they offer more space for the oxygen molecules to dissolve.
  • Waters with a lower concentration of nutrients and heavy metals have better DO than contaminated water.
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Contaminated waters promote harmful algae blooms and biofilms in the water, preventing the oxygen molecules in the air from reaching the water. Algae blooms also invite bacteria, which consume most of the DO, threatening aquatic life.

  • Clearwaters with sufficient photo periods have higher DO than contaminated waters. Low/No light penetrates through algal blooms, creating dead zones where no aquatic life can survive.

The below image shows the areas with different DO levels at the bottom of the Gulf of Mexico from 8th June to 17th July.2009. Zones in orange and yellow have lower DO, and green areas are well-oxygenated.

 Dissolved Oxygen (DO) Water Level
Image Credit: usgs.gov

We can draw parallels from the above to improve DO in aquariums.

Other Factors

To identify other factors, let us first understand a fish’s respiration system.

How Do Fish Breathe

Most fishes use their mouths, gills, and the gill flap/cover (known as operculum) to breathe DO.

Generally, fishes have four gills on each side, after their heads and below their eyes. Operculum is thick and bony. It doubles as a gill cover, besides helping in respiration.

The internal gill structure is broad, plate-like, with a lot of branching. Gills also have a network of nerves enabling the blood to flow to and from its organs, lending them a bright red color.

Fishes take water in their mouths while swimming. When their mouths are open, the operculum is closed. Water enters their gills through the gill walls.

The water and blood in their bodies flow in opposite directions, causing a movement (surface disturbance), enabling gaseous exchange. Both DO and CO2 flow from higher-concentration areas to lower-concentration areas. Thus, DO (from inhaled water) flows inwards in the body, while CO2-rich blood (from the body) flows outwards (but not outside).

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DO from the water in the gills is absorbed in the blood and circulated by the vascular system throughout their bodies.

When the fishes close their mouths, the gill flaps open, releasing the water consisting of CO2 from the bodies. While it seems lengthy, it takes only a few seconds to complete.

The below diagram demonstrates fish respiration.

How Fish Breathe - fish respiration system

There are exceptions, but since gill breathing is common in most fishes, we have covered it in detail.

Listed below are a few of them.

  • Some fishes (like Lungfish,Bettas, Gouramis, etc.) have a lung-like organ called a labyrinth, enabling them to breathe O2 from the air, in addition to gill breathing.
  • Sea Stars (Starfish) breathe through tiny tube feet below their arms/rays.
  • Frogs breathe through their skin.
  • Fish fry(s) also breathe through their skin till their gills develop.
  • Dojo loaches breathe through their anus, infusing DO into their intestines and releasing CO2 through the anus.

Other Factors Causing Fish Drowning

  • High nutrient levels in the water due to decaying matter at the bottom or excess CO2 injections/fertilizers/medicines in the water, etc.
  • Overfeeding will result in uneaten food, impacting water quality if uncleaned.
  • Too many aquatic plants in the community tank. The number of plants added should be proportionate to the tank size, else they will impact the water chemistry drastically, during respiration at night.
  • Insufficient lighting will result in plant respiration, reducing DO levels in the tank water.
  • Poor aquarium maintenance may suffocate fishes in the water or cause other complications leading to eventual drowning.
  • Overcrowded community tanks will lead to health complications/suffocate fish.
  • Harmful algae bloom due to high nutrient levels, or excessive photoperiod may lead to fish drowning. Refer to section 2.1 for more details.
  • Damaged gills while escaping fishing equipment may impact breathing. Impacted fish will either drown in the water or fall prey to predators due to weakened reflexes.
  • Since fishes breathe through their mouths and gills, they swim forward in the water. If a fish is trapped in a fishing net and pulled backward, it will use a lot of energy to free itself. In the process, it will need more DO. Plus, backward swimming is not its expertise, and it may suffocate.
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Besides, if the trapped fish cannot open/close its mouth due to the trap, it will be unable to breathe and die.

  • Many fishermen use fishing nets to trap sharks and pull them backward. They release these sharks into the water after cutting their fins. Shark fins are sold commercially for a delicacy called fin soup.

Sharks cannot swim backward due to their anatomy. They use their fins to direct water towards the gills and breathe. Without fins, they cannot respire properly. Finless Sharks drown in the bottom waters and die/fall prey to other fishes.

  • Some diseases in fishes (like gill rot, gill flukes, gill lice, swim bladder disease, etc.) may drown them unless addressed in a timely manner.
  • Less/No water current or oxygen supply in the aquarium. Goldfish in small bowls often drown to the bottom due to low DO.
  • Faulty/insufficient oxygen and filtration systems may deteriorate the water quality, drowning fish.

Why Don’t Fishes Drown When They Sleep

Diurnal fishes sleep at night, while nocturnal fishes sleep during the day. They cannot close their eyes, as they do not have eyelids.

Depending on the species, they prefer a hideout (cave, plant, rock, etc.) or rest on any object. Some fishes remain afloat at a certain water level. They maintain buoyancy at the desired water level by increasing/decreasing the gas inside their swim bladders. Fishes without swim bladders relax on objects. Some swim extremely slowly when asleep.

Hence, fishes do not drown in the water while sleeping, unless there is a sudden DO drop, a spike in CO2, etc. This may happen in the wild due to a natural calamity (volcano eruption, chemical discharge in the water, etc.).

Final Thoughts

Proper aquarium maintenance is the key to protecting your pet fish from drowning in the water.

Avoid overcrowding and overfeeding. Choose a suitable aquarium setup and decor with ample free/hiding spaces and an appropriate number of plants, if any. Invest in an oxygen and filtration system. Perform weekly partial water changes with quality replacement water.

Switch off the aquarium lights at night. Choose lighting that nurtures your plant without stressing the tank dwellers.

Are you ready to bet with your family, friends, or colleagues about it? We wish you all the best!

About the Author

Shelby Crosby

Shelby is a passionate fishkeeper who has been writing about fish for over 5 years. She is a pro aquarist and holds a BSc Honors Degree in Wildlife and Fisheries. She creates her own beautiful aquarium layouts and loves to share her knowledge of tropical fish with other hobbyists. Career Highlights: Has worked with several…

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