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Can Fish Drown In Water? Truth Or Myth?

Most fish spend the majority of their lives in water; everyone knows that. 

But can fish drown in your fish tank? Do fish have lungs? How do fish get oxygen? And can you drown a fish by pulling it backwards through the water? 

In this guide, we explain what steps you can take to prevent cases of accidental suffocation and drowning of fish in your aquarium.

Do Fish Need Oxygen To Survive?

As humans, we need to breathe air to obtain the oxygen that we need to survive. Fish need oxygen too, but they collect the oxygen they need from the water. And, as we do, fish exhale carbon dioxide.

How does that work?

Rather than breathing atmospheric air, your fish absorb dissolved oxygen from the water through their gills. Like humans and other mammals, fish need oxygen for many of the body’s processes, including creating proteins for building new cells and breaking down nutrients into usable energy.

Do Fish Have Lungs?

So, now you know that fish need to take in oxygen and expel carbon dioxide to survive. However, instead of lungs, most fish have gills.

Gills are branched organs that are located on each side of the fish’s head. Many fish have four gills on each side of their body that are formed by interlinked columns of cells.

The gills contain a great many tiny blood vessels called capillaries. As the fish swims, it opens its mouth, allowing the water to flow over the gills where the capillaries absorb the dissolved oxygen from the water. The capillaries then circulate the oxygen throughout the fish’s body to the organs and other structures that require it.

How Do The Gills Work?

Different fish species’ gills work in different ways. Most fish push water across their gills by opening and closing their mouths that has the effect of widening the gill covers. As the bottom of the fish’s mouth drops, the gill covers open. As the fish closes its mouth, water is caught within the gills, and oxygen is extracted from it. The water is then expelled through the operculum.

Other fish species, such as rays and sharks, have five-gill slits on each side of the body supported by a rigid cartilaginous arch that keeps the gills from collapsing. Unlike most fish, sharks generally have to swim continuously forward to force water over their gills. If the shark stops swimming, it suffocates.

Salty Problem

Too much salt in the bloodstream can interfere with the fish’s ability to exchange oxygen and carbon dioxide. For that reason, marine fish are able to expel excess salt from their body. 

On the other hand, freshwater fish have the opposite problem in that they need to maintain adequate levels of salt in their body. So, to maintain homeostasis, freshwater fish have chloride cells that regulate their salt balance.

What Are Lungfish? 

There is a small group of several freshwater lungfishes that are able to breathe air by using lungs rather than gills.

These fishes were once common during prehistoric times, but are now restricted to areas of Australia, Africa, and South America. These fish have evolved to survive during dry seasons when the water in their habitat dries up by burrowing down into the substrate and entering a semi-hibernation state until the rains come again.

During this time, the fishes’ metabolism slows down to just 1/60th of its usual rate!

The Labyrinth Organ

As well as lungfish, there are over 370 species of fish that can breathe air. Those fish are categorized as obligate and facultative fish.

Facultative air breathers only breathe air when the water is oxygen-depleted. Obligate air breathers need to breathe atmospheric air because their gills aren’t able to extract enough oxygen from the water.

Obligate air breathers are also known as Anabantoidei or labyrinth fishes, a group of fish that includes popular aquarium species such as gouramis and bettas. Labyrinth fishes can take oxygen directly from the air via their labyrinth organ. These fish have to rise to the surface periodically to take gulps of air. That air is forced into the labyrinth organ, where the oxygen is extracted from it.

Fish are not born with a fully formed labyrinth organ. That anatomical feature develops slowly as the fish matures.

How The Labyrinth Organ Works

Mini graphics of anatomy of a fish

Inside the fish’s labyrinth organ there are lots of little maze-like structures made up of thin bony plates called lamellae. Oxygen passes through the thin membranes that coat the lamellae into the fish’s bloodstream.

During the drought season, if the water dries up, the labyrinth fish can survive for quite a long time, provided its skin is moist. Some species of labyrinth fish can crawl across dry land to reach water, others achieve the same thing by jumping, and one fish, the Climbing Perch, can even climb trees.

You should know that labyrinth fishes must breathe atmospheric air to survive. If the fish don’t have access to the water surface to breathe, some species will die. Also, the ambient temperature in the room where you keep your tank must be the same as that of the water, otherwise, the labyrinth organ can be damaged.


What Are The Signs Of Low Oxygen In The Water

So, you know that all fish species extract some of the oxygen they need directly from the water. Depleted oxygen levels can cause big problems for fish. If there’s not enough oxygen available in the water, the fish can suffocate or drown.

If the oxygen levels in your fish tank are too low, your fishes’ behavior will tell you that there’s a problem. Signs of low oxygen levels in the water include:

  • Reduced activity levels
  • Poor appetite
  • Labored breathing and more rapid gill movements
  • Gasping at the water surface

Although labyrinth breathers will frequently go to the water surface to take gulps of air, fish that are struggling to take sufficient from the water will gasp continually at the surface, usually with an open mouth.

What Causes Low Oxygen Levels In Your Aquarium?

There are several causes of low oxygen levels in a home aquarium, including:

  • Overcrowding
  • High water temperature
  • Poor water movement
  • Excess waste
  • Live plants with low lighting
  • Use of certain chemicals

However, the primary cause of fish suffocation is almost always caused by overcrowding. 

The larger the tank, the more oxygen it will contain, simply because the water volume is greater and the large surface area offers more scope for efficient gaseous exchange. In a small fish tank, the amount of dissolved oxygen will be less, especially if the aquarium is overstocked with too many fish that are all competing for what oxygen is available.

Breathing Underwater Is Difficult – Even For Fish!

Mini graphics of fish respiratory system

Cool water typically holds more oxygen than warm water. So, a tropical tank may well contain less oxygen than a coldwater aquarium. 

Also, other organisms that live in your tank use oxygen, including plants, algae, and bacteria. During the daytime, algae and plants do give off oxygen during photosynthesis. However, at night, photosynthesis stops, and plants and those other organisms continue to respire, using up valuable oxygen.

Breathing underwater is hard work! The atmospheric air that humans breathe contains 200,000 ppm (parts per million) oxygen. However, water offers only 4 to 8 ppm. So, your fish must pass a relatively massive amount of water across their gills to extract enough oxygen to survive. 

As fish are cold-blooded, they need less oxygen than warm-blooded animals, and fish also have a slower metabolism than warm-blooded animals so, consequently, they need less oxygen. 

Can Water Contain Too Much Oxygen?

Tropical and aquarium goldfish in blue water. Beautiful background of the underwater world

It’s virtually impossible for a fish tank to contain too much oxygen unless the environment is placed under pressure, which is almost never the case with a home aquarium. 

Any oxygen that you pump into the water via a bubbler or airstone will simply evaporate into the atmosphere. 

You might have heard of a condition called “gas bubble disease” that causes tiny bubbles of gas to enter the fish through the gills. However, gas bubble disease is pretty rare and is generally caused by nitrogen in saltwater environments rather than in freshwater tanks that are well oxygenated.

How To Improve Oxygen Levels In Your Fish Tank

There are a few ways of improving the oxygen levels in your fish tank:

High Water Temperature

Since a high water temperature cannot hold as much oxygen as cooler water, you can help to oxygenate the water by performing a water change with water of a lower temperature. However, you must be careful to keep the temperature within the parameters that your fish can tolerate.

If the tank becomes too warm, turn off the heater and the lights. Remove the aquarium lid or cover slide, and use a fan to blow cool air across the water surface. You can also add a zip-close bag containing a few ice cubes to help lower the water temperature.

If you live in a warm climate, adding an aquarium chiller to your setup might be helpful.

Increase Water Movement

Stagnant water has very poor oxygen levels, and that applies particularly to the lower areas of the tank where there’s little water movement.

Although there will be more oxygen available at the water surface due to gaseous exchange, that oxygen doesn’t get to the lower areas of the water column. So, it’s essential that the water is circulated properly to move the water from the bottom of the tank to the top, displacing the oxygenated surface water further down into the aquarium.

An efficient filtration system will help to keep the water oxygenated by drawing water into the bottom of the filter and then recirculating it back through the water at the top of the aquarium. So, if you keep your filter media clean so that the water can flow freely through it, you’ll be helping to keep the water moving around the tank.

If water circulation is poor, you might want to consider buying a more powerful filter unit. Alternatively, try installing a spray bar on the filter outlet or adding an air stone or bubbler.

Too Much Waste

Aquarium with fishes on dark background

Overstocking causes problems with too much waste in the tank. The more waste there is, the more likely it is that you’ll have problems with excess algae growth and clogged filters. As well as poor oxygen saturation levels, too much waste will reduce the oxygen-carrying capacity of your tank.

As the beneficial bacteria in the tank work to break down organic waste trapped in the substrate, they use lots of dissolved oxygen. If you clean your gravel thoroughly to remove debris, that can prevent the problem.

Living Plants

Although living plants are undoubtedly beneficial in freshwater tanks, too many can cause problems with oxygen levels in the water.

During photosynthesis, plants absorb carbon dioxide and release oxygen. However, that only happens when the plants are exposed to lights. When the lights are turned off, the process is reversed, and the plants and algae take up oxygen from the water.

Pro Tip 

The easy solution here is to increase the length of time you have your tank lights on.

Chemicals

If you use chemical treatments in your tank as medication or to adjust your water parameters, check the product label carefully. Some of these chemicals can affect the water’s oxygen-carrying capacity.

You can combat that effect by increasing the water circulation while you’re medicating your fish.

Can Fish Actually Drown?

So, now you know that fish can suffocate if their tank doesn’t contain enough dissolved oxygen in the water, and labyrinth breathers can die if they’re deprived of access to the water surface to breathe.

But can fish actually drown in water?

No! Fish cannot drown in water. 

Can Fish Drown If They Are Pulled Backward?

The answer to that question is, yes, in theory, a fish could drown if it was pulled backward through the water.

How so?

Remember how much water needs to pass across the fish’s gills for the creature to be able to absorb enough oxygen to survive? Well, if the fish was dragged backward through the water, the gills would be unable to process sufficient amounts of oxygen. So, the fish would effectively suffocate. However, most fish would finish up drowning simply because they would be unable to maintain the effort.

Also, as mentioned previously, fish that push water through their gills by swimming forward will drown or suffocate if they’re prevented from doing that. So, if a shark, for example, was to become trapped and unable to swim, it will suffocate. That’s why some shark hunters drag sharks back toward the shore to kill them by suffocation.

Gill Damage

If the fish’s gills are damaged as a result of trauma, disease, or interference in gill function by chemicals in the water, it’s possible for the fish to suffocate, even if there’s plenty of dissolved oxygen in the water.

As previously mentioned, breathing underwater is extremely hard work compared with breathing atmospheric air. So, if the fish has damaged gills, it will struggle to take up enough oxygen to survive unless the fish is a labyrinth breather and can breathe at the water surface. 

Can Fish Survive In Milk?

Skim milk is composed of roughly 90.4% water. So, in theory, a fish should be able to survive in milk, right?

Well, actually, no. A fish would not survive for very long in milk. Here’s why …

A bowl of milk

Fish live in water that contains dissolved oxygen, a few trace minerals, and some acidity. Although skim milk is made up of nine-tenths water, the low levels of oxygen and acidity, as well as the fat, carbs, proteins, and other minerals that milk contains, would certainly clog up the fish’s gills. In all likelihood, the poor fish would be dead within a matter of minutes if it was put into a bowl of milk.

So, essentially, skim milk is basically the same to the fish as very heavily polluted water.

Do Fish Feel Pain?

According to a recent review carried out at Liverpool University, UK, fish almost certainly do feel pain, although probably not in the same way that mammals do.

Research shows that fish do have nociceptors (pain detecting cells) in their mouths. However, although fish do feel pain, they respond to it differently from other animals, and the way in which fish feel discomfort is different, too. For example, fish are cold-blooded animals that are not as sensitive to the cold as most mammals. However, fish are highly sensitive to pressure.

Goldfish in an Aquarium

During the research, veterinary studies showed that the degree of pressure necessary to stimulate the nociceptors is much lower in fish than it is in mammals. In fact, the sensation in fish is very similar to that of the human eye. So, you can see that handling your fish is likely to cause them pain.

In Conclusion

I hope you enjoyed our article.

Fish don’t generally drown in water. However, your fishy friends can suffocate if the levels of dissolved oxygen in your tank are too low.

To ensure that oxygen levels remain sufficient for your fish, don’t overstock your tank, maintain your filter media properly, monitor the tank temperature so that it doesn’t get too warm, and consider using an air stone to boost water movement and surface agitation.

If you have any questions, please put them in the comments box below, and don’t forget to share this guide if you found it interesting and helpful.

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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