Can Fish Drown? The Surprisingly Complicated Answer

Whether it be from a new aquarist or someone trying to win a trivia game, “can fish drown” is one of the most common questions we get asked. This guide will help you understand if it’s possible, and help you understand the process.

Can Fish Drown?

Most people assume that fish are incapable of drowning. After all, they live and breathe underwater!

This may come as a surprise to many, but fish can easily die from a lack of oxygen. Believe it or not, fish require oxygen to survive just like humans and air-breathing animals. When they don’t have access to usable oxygen, it’s impossible for them to stay alive.

You see, oxygen is responsible for sustaining both land and aquatic life. Oxygen combines with other elements to create proteins and living cells. It also plays a part in storing energy that every biological function in a fish’s body uses to stay active.

Oxygen supports the organs and ensures that everything from the brain to the liver is working correctly. 

A fish in a tank where they can't drown

While dramatically different on the surface, there are several similarities between your body and the body of a fish. Both have a heart that continually pumps oxygenated blood to various organs. Both also have organs that exchange oxygen for carbon dioxide.

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The difference lies in how fish respirate and how they get their oxygen.

Fish take advantage of dissolved oxygen in the water. You can’t see oxygen molecules, but they’re there nonetheless.

The oxygen concentration in water is between 2 and 8 PPM, or parts per million. For comparison, the air we breathe is about 210,000 PPM.

Fish clearly have a more challenging job of getting oxygen. Fortunately, they don’t need as much as we do. Most fish are cold-blooded. They can get by without using all the extra energy warm-blooded animals use to stay alive.

Therefore, they can survive on much lower oxygen levels. Even still, fish still need oxygen to live. 

So, can fish drown?

In a sense, fish are very capable of drowning in low-oxygen environments. But technically speaking, the correct term for what occurs wouldn’t be “drowning.” What happens is closer to the act of suffocating.

Author Note: To drown is to die by inhaling water while submerged. Meanwhile, to suffocate is to die from the lack of air or inability to respire. When you look at the biological processes fish use to survive, suffocation is the more appropriate term.

When we refer to fish “drowning” in this article, that’s simply us using the common terminology for the benefit of new owners. For those with a bit more experience, know that we really mean suffocating.

The Organs Fish Use To Breathe

The anatomy of a fish is complex and highly varied from fish to fish. That said, the breathing system remains the same across most species. Understanding this is important if you want to understand how fish can drown.

But before we get into the organs that most fish use to breathe, let’s go over some outliers.

There’s a small group of six freshwater fish species that have air-breathing lungs. They’re appropriately called lungfish. 

Lungfish used to be far more common. Several millennia ago, they were widespread and appeared in massive networks of freshwater lakes and rivers. 

However, only a small population now remains. Thanks to their air-breathing nature, lungfish can adapt to the changing conditions of drying lakebeds. They burrow in the substrate and enter a state of quasi-hibernation to survive dry spells.

While lungfish are quite rare, fish with a labyrinth organ are easy to find. In fact, many species with this unique organ are popular in the aquarium trade!

One gourami labyrinth fish

Labyrinth organs act like modified lungs. They allow fish to take in atmospheric oxygen by swimming to the surface and taking a gulp (meaning that drowning would have to be caused a bit differently). The most popular labyrinth fish are the betta fish and different types of gourami.

Most fish species rely on a set of gills to breathe. The gills are a set of specialized organs on each side of the head. As your fish respirate, you can usually catch a glimpse of them.

The gills look like arches of red. Tiny filaments and lamellae create an appearance that many compare to a car radiator. Most species have four gills on each side of the body. However, this number can vary a bit from one species to the next.

Regardless, the gills are responsible for a complex exchange of gases that help fish utilize dissolved oxygen for survival.

How They Breathe

The respiration process for fish is complex and fascinating. The gills are highly effective organs that are capable of pulling even minute levels of dissolved oxygen out of the water for use.

To access the oxygen in their surroundings, fish have to force water through the gills. To do that, most utilize a small flap of skin called the operculum. The operculum is that little slit that opens and closes as the fish respires.

When they “take a breath,” fish will adjust the pressure in the mouth to move water. Lowering the floor of the mouth causes water to flow in. Raising the floor then pushes the water out through the operculum.

As the water flows past the gills, the feathery filaments extract dissolved oxygen through a process called countercurrent exchange. The epithelium tissue then facilitates oxygen absorption into the bloodstream.

Because oxygen levels in the water aren’t anywhere close to what they are in the air, fish need to move a lot of water to stay alive. The large surface area of the gills helps them collect as much oxygen as possible. If they’re unable to do this, they can drown (although we know it’s technically more like suffocating).

Reasons For Low Oxygen Levels In The Water

Now that you understand how they breathe, let’s go over some reasons that a fish might drown/suffocate. Even in a well-maintained tank, there’s only so much oxygen to go around. Oxygen levels must regularly replenish to keep fish alive.

It’s when oxygen levels fall too low that fish can drown. Anything below 2 PPM is considered the danger zone. Here are some of the most common reasons why oxygen levels fall to dangerously low levels.


There’s a reason why aquarists always warn novices about overcrowding. It’s not just a matter of fish comfort and stress. Packing too many fish into a confined space is just asking for trouble.

As mentioned earlier, there’s only so much oxygen to go around. Fish need to utilize as much water as possible to collect the oxygen they need to survive. When you have too much fish in the habitat, the available oxygen levels deplete pretty quickly.

Before you know it, you’ll notice labored breathing and slow movement from the fish. Eventually, the fish will start dropping one by one due to drowning or suffocation.

Author Note: Avoid overcrowding at all costs. Read up on the space requirements for each fish and try to get the biggest tank possible. If you’re in doubt, take some fish out and keep them in another tank! It’s better to have less fish density than to create potential health problems.

Algae Blooms

Algae blooms can be a nightmare to deal with. They seem to happen out of the blue and can quickly turn your aquarium into a green mess.

Unfortunately, algae blooms are detrimental to your fish’s health, too. Bacteria populations proliferate rapidly. The algae consumes all of the usable oxygen as it dies and decomposes, leaving very little for your fish to use.

Even when the algae eventually die off, the oxygen levels take time to recover. Blooms dramatically change water chemistry. Some types of algae can even produce fatal toxins!

Author Note: Algae blooms usually occur due to excess nutrients. In many cases, it’s a product of overfeeding.

High Water Temperatures

Many new owners who ask, “Can fish drown?” tend to forget about this potential cause. It’s always important to stay on top of water parameters like temperature. Not only do significant fluctuations cause stress in fish, but they can also alter the oxygen levels.

Warmer water holds less dissolved oxygen than cool water. Hotter surface temperatures make the gas exchange between the water and air less efficient, which will change the water chemistry.

Poor Water Movement

In nature, water becomes oxygenated because of continual movement. The crashing of waves, splashes, and underwater flow will infuse the water with oxygen. Surface movement is crucial!

That’s how aquariums replenish the supply of dissolved oxygen. Your filter works to churn the surface and keep things moving.

If the water is too stagnant, lower oxygen levels are sure to follow. 

The lower parts of the water column usually experience the ill effects first. The flow from your filter return helps to churn the water and distribute oxygen evenly throughout the tank.

Once that stops, the surface may hold some oxygen. However, bottom feeder fish will suffer.

Author Note: Fortunately, fixing this problem is pretty straightforward. You can clean your filter, add another pump, or use an air bladder. Anything that moves the water will help increase oxygen levels.

Small Water Surface Area

This point piggybacks on the previous one. As we stated earlier, movement on the surface plays a big part in creating dissolved oxygen throughout.

If your tank is small or lacks adequate surface area, the oxygen levels can plummet dramatically and result in your fish drowning.

This is why it’s so dangerous to keep fish in small bowls or vases. Labyrinth fish, such as the betta, can get by because they breathe atmospheric air. But standard fish will suffer from low oxygen levels right off the bat!

If you do use a small bowl, make sure that there’s an air pump and bladder to oxygenate the water.

Too Much Plant & Animal Waste

Excess waste can have a terrible impact on your tank. The longer plant and animal waste sit in the water, the farther along the decomposition process will go.

While that might not seem like a huge deal, decomposition requires dissolved oxygen. Like a flourishing algae bloom, too much decay will deprive your fish of some much-needed oxygen.

Keep the tank clean and use a gravel vacuum to remove waste from the tank. It’s a good idea to perform regular water changes, too.

Lighting & Plant Life

Plants are a fantastic way to infuse a fish tank with some supplemental oxygen. However, they’re only effective when you provide the plants with a conducive growing environment.

One of the biggest mistakes newer aquarists make is ignoring lighting requirements. Aquatic plants need light to grow and photosynthesize. That way, they can utilize carbon dioxide and produce dissolved oxygen.

If you attempt to grow plants in a dark tank, the plant will become an oxygen hog. It goes into survival mode and starts consuming oxygen to survive. 

Plants that grow in subdued lighting for long periods can use up enough oxygen to negatively impact your fish. Luckily, the solution is easy. Just provide more lighting!

Chemical Treatments

Finally, oxygen levels can change due to chemicals in the water. Some substances will alter the water’s ability to carry dissolved oxygen (causing your fish to drown or suffocate). 

Make sure to read warnings on all potential water additives.

If you plan on using chemical-based fish medications, consider increasing water circulation to be on the safe side. Use an air pump to overcome any potential water chemistry side effects.

Wrapping Up

Now that you know the answer to the question, “can fish drown” it’s your job to make sure you provide an environment that is safe and healthy. Low oxygen levels can occur in a tank without much warning, so it’s important to keep an eye on things!

We hope this guide was informative and helped answer your question. If you need us to clear anything up, just send over a message.

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