Can Fish Drown – A Short and Long Answer

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Many people have heard the saying that fish can drown, but may not be sure what it means. In this post, we will explore the short and long answer to can fish drown. We will also look at some of the reasons why fish might drown and what you can do to help prevent it. Stay tuned for more information on this interesting topic!

The Short Answer

Yes, fish can drown. However, it’s better to think about a fish drowning as the result of a lack of oxygen instead of seeing their lungs fill with water.

Fish can be deprived of oxygen due to several reasons, including oxygen levels in the water, poor water quality, parasites and diseases, and even physical deformities.

In short though, fish can drown in water due to not being able to extract the oxygen they need from their surroundings.

The Long Answer

It isn’t every day that you hear about a fish dying due to drowning. This lack of oxygen can be due to several biological, chemical, and pathological problems within the aquarium including poor water quality, parasites and diseases, and deformities.

First, we need to understand how oxygen enters the aquarium and how fish breathe. Then we can understand how the transfer of oxygen from the water to the bloodstream of the fish can fail.

Dissolved Oxygen In The Aquarium

Air Bubbler

Dissolved oxygen, often abbreviated as DO, is how much oxygen is available in water in comparison to how much oxygen that water can hold; simply put, dissolved oxygen shows how much oxygen is available in the water to be used by plants and animals at any given time.

While dissolved oxygen levels remain pretty consistent in the aquarium setting, this scientific parameter can be affected by several factors, including water temperature and depth, surface exchange, photosynthesis and respiration, and fish medications.

Water Temperature And Depth

Water temperature and depth are the main factors that influence dissolved oxygen levels in bodies of water.

Due to the physical properties of water, cold water holds more oxygen while hot water holds less oxygen. This means that the warmest surface waters, that aren’t affected by surface exchange, hold less oxygen than the coldest water at the very bottom of the ocean. Other external factors, such as seasonal, altitude, and latitude differences, all play a role in how much oxygen is actually available at any given time.

In the aquarium, the water temperature can definitely affect dissolved oxygen levels. For instance, if you need to raise water temperatures to treat a parasite, like ich, it is recommended to increase surface agitation and to add an air stone and air pump. This is because the warmed water will hold significantly less oxygen, causing your already-sick fish to have even more difficulty breathing.

Though it is likely for fish to succumb to the shock of rapidly changing temperatures first, it is possible for fish to drown if oxygen levels are low in warm water.

Surface Agitation

Surface agitation is one of the ways that oxygen enters the water column. This is a physical exchange between atmospheric oxygen and water through wind and wave action. Other gases, such as carbon dioxide, are also exchanged at this point to create a cycle.

In the aquarium, surface agitation is very important. Filters, powerheads, and air stones are common methods of increasing surface agitation to help increase dissolved oxygen levels and to improve overall gas exchange. If surface agitation is poor in the aquarium, then dissolved oxygen levels will also be low.

This is a very common problem in betta fish (Betta splendens) aquariums. These labyrinth fish are very delicate and need little to no water flow in order to prevent injury. As a result, there is usually minimal surface agitation, leaving the betta to sometimes rely on its labyrinth organ; the labyrinth organ is a specialized lung-like organ that allows the fish to process atmospheric air directly.

Photosynthesis And Respiration

Photosynthesis is the process by which an organism converts carbon dioxide into oxygen in order to make food. Respiration is the opposite of this process where oxygen is converted back into carbon dioxide.

During the day, aquatic plants and vegetation perform photosynthesis while fish perform respiration. At night, both plants and fish respire. Together, this ecosystem works as a constant exchange between carbon dioxide and oxygen.

In freshwater and saltwater systems, autotrophic plankton called plankton is responsible for producing most of the available oxygen in the world. However, an imbalance between photosynthesis and respiration can lead to problems and fish drowning.

One of the most damaging events to happen in nature is a harmful algae bloom (HAB). Harmful algae blooms happen when nutrients and resources are abundant, usually due to excess fertilizer and runoff, leading to exponential growth. This growth can be seen in algae and some harmful bacteria, like cyanobacteria, that can produce deadly toxins.

When algae and bacteria populations are left to grow out of control, there becomes an imbalance of resources. Eventually, sunlight is blocked while nutrients are depleted, depriving the plants below of performing photosynthesis and putting oxygen back into the system. Once these plants die, the decomposition of the organic matter also increases nutrient and respiration rates, fueling growth and further depleting oxygen.

As this happens, oxygen levels are incredibly low and most fish and invertebrates suffer and die. If the harmful algae bloom is caused by cyanobacteria, they may be affected by deadly toxins as well; these toxins are why some bodies of water are off-limits to people during harmful algae blooms.

Harmful algae blooms to this extreme are not likely to happen in the aquarium, though strains of harmful cyanobacteria do exist in the hobby. Instead, it is possible to have too many nutrient-hungry plant species that quickly overrun the aquarium and then die off when resources are depleted. This die off can have similar effects where the decomposition quickly removes oxygen from the water.


spectrogram Medication

Many medications for freshwater and saltwater fish advise increasing aeration during the treatment period. This is due to how the medications interact with the water chemistry. Effects can be enhanced if mixing medications.

Not all medications will lower the oxygen level in the tank, but it’s definitely worth running an air pump if it is advised.

How Do Fish Breathe?

Before we see how fish drown, we need to understand how they breathe first. Here is a great video below by Steve Griffins. I’ll explain more below.

Most fish breathe through their gills; many fish have made adaptations to combat lower oxygen levels, though we will only look at standard fish anatomy. Gills provide a large and highly-folded surface area with an extensive network of capillaries that can be used for the exchange of gases, like oxygen and carbon dioxide.

In order to breathe, fish pull water into their mouth which is then pushed across their gills. Oxygen is absorbed and diffused into the bloodstream and transferred throughout the body due to differing concentrations; the concentration of oxygen is lower in the blood than in the surrounding environment.

Carbon dioxide is then moved across the gills, out of the body of the fish, and back into the water.

Can Fish Drown?

Now that we know what affects dissolved oxygen levels and how fish breathe, we can see where the process can go wrong. Possible problems include poor water quality, parasites and diseases, and physical deformities.

Some of the telltale signs of a drowning fish are:

  • Heavy breathing
  • Red or inflamed gills
  • Laying at the bottom of the aquarium or gasping for air at the surface

Poor Water Quality

Poor water quality can be a secret killer of fish, especially if water parameters aren’t being regularly tested. Bad water parameters, namely high levels of ammonia and nitrite, can make breathing very difficult and very painful for fish.

It should be noted that high levels of nitrate can also prove to be deadly to fish and invertebrates but is not as common of a problem as the other parameters.

Ammonia Poisoning

Ammonia poisoning happens when there are especially high levels of ammonia present in the aquarium; any level of ammonia past 0.0 ppm can be damaging to fish and invertebrates, but serious damage starts to happen past 0.5 ppm.

Ammonia is toxic and will burn the internal and external organs of fish, including gills. This can quickly lead to fish being unable to pull enough oxygen across their gills, leaving them to suffocate and drown.

High levels of ammonia are usually caused by having too many fish, excess feedings, or stocking an aquarium that is not fully cycled.

Nitrite Poisoning

Nitrite poisoning is just as deadly as ammonia poisoning and can be a much more silent killer. Again, nitrite levels should always be 0.0 ppm. Any amount past this can start to cause damage to internal and external organs.

Nitrite poisoning happens when there are excess levels of nitrite in the aquarium. Due to the chemical structure of hemoglobin, nitrite can bind to blood where oxygen would otherwise. This means that your fish will suffocate from the inside out, making for a very painful death.

High levels of nitrite are the result of high levels of ammonia; any trace of nitrite is indicative of a nitrogen cycle happening in the fish tank.

Parasites And Diseases

Unfortunately, many parasites and diseases that are in the fish tank target the gills of fish. Luckily, most parasites and diseases present themselves before there is irreversible damage, but some ailments might be quicker and more difficult to treat than others.

One of the worst parasites to encounter are species of flatworm, particularly from the Monogenenea class.

Gill Flukes

Gills flukes (Dactylogyrus spp.) are one of the most dangerous parasites that can make fish drown. These flukes are most likely to occur on members of the Cyprinidae family, including goldfish and koi, though they can infect any freshwater fish species.

Gill flukes are initially free-swimming parasites that quickly need to find a host. At that point, they attach to the gills of the fish and reproduce. The gills will then become inflamed and your fish may try to scratch itself against hard surfaces to try to get them off. This can lead to a secondary infection, making treatment even more difficult.

A gill fluke infection often presents itself as red and inflamed gills, lack of appetite, scratching, and heavy breathing. Parasitic-specific medications, like Praziquantel, will need to be dosed to eradicate the flukes.


Freshwater Ich

Ich (Ichthyophthirius multifiliis) is one of the most common parasites to come across in the aquarium. Though these organisms do not target fish’s gills specifically, they can infect the gills which can cause the fish to drown; unfortunately, there are a few different types of ich that will only affect gills and leave the rest of the body alone, making the initial diagnosis difficult.

Otherwise, ich is easy to diagnose and presents itself as tiny white spots that cover the body of the fish. Other symptoms are similar to those found with gill flukes.


Lastly, some physical deformities can cause aquarium fish to drown, specifically betta fish.

Betta fish have been bred to have the most extreme colors and finnage. While beautiful to look at, some of these modifications have turned into deformities that hinder the fish from being able to breathe air.

Though betta fish have gills that they can use to get enough oxygen from the water, they will still venture to the surface of the fish tank to take in atmospheric air regardless of water parameters. However, some deformities, like excess finnage, can drag your labyrinth fish down, never allowing them to get the atmospheric air they need.

At the same time, your fish may become exhausted while trying to swim to the top of the tank. This activity will cause your fish to breathe harder, which will cause carbon dioxide levels to rise. Though this only happens in extreme cases, it’s possible for your betta fish to over-exhaust itself and not get enough oxygen from the water as a result.

Does Your Aquarium Have Enough Oxygen?

As long as the water is flowing in the aquarium and regular tank maintenance is maintained, there’s usually no reason to worry about adding additional aeration to the system. However, problems may arise when too many fish are kept in too small of a tank or if water movement is inadequate.

How To Improve Aquarium Oxygenation

The best ways to improve oxygenation in the aquarium are through equipment, tank maintenance, and photosynthetic additions.

In general, it’s recommended to choose a filter that is rated for at least double the size of your aquarium. Not only does this allow for more beneficial bacteria growth, but the subsequent surface agitation is a great source of oxygenation.

Freshwater tanks need at least a 6-10x turnover rate; heavily planted tanks will need better circulation to ensure that water can pass through. Saltwater reef tanks may need at least a 20x turnover rate with specialized tanks reaching much higher.

In addition to filtration, powerheads and wavemakers can be added to the freshwater or saltwater aquarium. If using external filtration, water will also be oxygenated as it moves from the tank and through the plumbing. Protein skimmers may also be added to the saltwater aquarium for additional filtration and oxygenation.

While filtration and equipment will help create surface agitation and distribute oxygen throughout the tank, corals and plants may also be added for additional photosynthesis. Fast-growing plants will quickly take up nutrients, releasing oxygen into the water in the process. Symbiotic algae within coral are slightly slower to photosynthesize, but they will contribute to oxygen levels while helping take up excess nutrients as well.

Regular tank maintenance will also introduce new oxygen and help accelerate gas exchange. Though this is a temporary effect, water changes are very effective for aerating and circulating new water throughout the tank.

Final Thoughts

Believe it or not, fish can drown. There are many factors that determine oxygen levels in the aquarium, including water temperature and depth, surface exchange, photosynthesis and respiration, and medications. Water quality, parasites and diseases, and physical deformities can further make it difficult for fish to breathe, leaving them to drown.

If you’re not sure about how much water flow you should have in your aquarium, add as much as you can without disturbing the substrate or stressing out fish and invertebrates.

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