If you’re new to fish keeping or just have a hankering to learn some interesting facts today, you might have come across the question: can you drown a fish?

You know that fish need oxygen to breathe – else what would be the point of all those pumps and filters and things? – but you also know they have gills and happen to, well, live in water.

This might also lead to the question: why do fish die out of water?

We’re going to answer these questions for you today.

can fish drown?

Is It Possible for Fish to Drown in Water?

Can fish drown in water?

Very simply put, NO, fish can’t drown in water.

However, they CAN suffocate.

The difference here comes down to the definition of the two words.

The dictionary says that drown means to “die through submersion in and inhalation of water.”

Only applicable to air-breathing animals.

Suffocation, on the other hand means “to kill by preventing the access of air to the blood through the lungs or analogous organs, as gills.” (emphasis mine)

Fish require oxygen to breathe, just as humans and other mammals do, but not in the same way. Humans and cats, for example, have lungs.

If mammal lungs are filled with water, they drown.

Fish have gills that are supposed to fill with water, but with the purpose of extracting oxygen from the water.

When fish cannot extract oxygen from water, they suffocate.

Since fish can suffocate in water, many people just call this drowning…though as we’ve just explained, it’s not technically true.

Facts About Fish TO Further Explain Drowning vs. Suffocating

  1. Can a fish drown? Not technically, but they can suffocate in water.
  2. Fish need oxygen levels in their watery home to be two parts per million – or more – to survive. Without it, they suffocate (what some people refer to as drowning).
  3. Fish do not have lungs – they have gills and other organs that function as their respiratory system.
  4. Fish use their gills to pull the oxygen from the water, which then allows them to breathe.
  5. Fish are cold-blooded (only one warm-blooded fish has been discovered) and therefore have a slower metabolism than mammals. This means they need less oxygen than warm-blooded creatures.
  6. Fish are more likely to suffocate in aquariums or fish tanks than in their natural habitats.

How Do Fish Suffocate (Drown) in Water?

Next, let’s take a look at the actual process through which a fish can suffocate in water and “drown”.

As we’ve said above, how fish suffocate or “drown” is through the lack of enough oxygen in the water in which the fish lives. These depleted oxygen levels in the fish tank happen because of a number of reasons.

  1. Too many live plants create too much oxygen
  2. Overcrowding of the fish tank (too many fish and invertebrates)
  3. Too little air movement and flow in the aquarium
  4. Limited water surface area

Fish, like humans, dispel carbon dioxide when they breathe. When they live in an aquarium full of other fish, with a limited water surface area, or in fish tank that doesn’t allow for air flow, that carbon dioxide can build up and get trapped inside the aquarium. This means, oxygen is depleted and carbon dioxide becomes a poison to them – just as it does to us.

Another reason a fish may drown is if his gills are damaged or diseased. Just like humans and mammals need healthy lungs to breathe well, fish need healthy gills.

Finally, there are some species of fish that rely on ram ventilation, which means swimming forward to push water through their gills. If they get caught and cannot swim forward, they stop bringing in oxygen and will expire that way.

How Do Fish Gills and Their Other Organs Work?

fish anatomy diagram

The very basic understanding we have of how this process works looks like this:

  1. Fish take in water through their mouths in gulps.
  2. The water passes through their feathery gill filaments.
  3. The gills extract oxygen from the water. This is called the countercurrent exchange.
  4. The oxygen is absorbed into the bloodstream.
  5. The water then passes out the other side of the gills back into the rest of the body of water.

In order for a fish to absorb the oxygen from the water, the oxygen level in the water must be higher than the oxygen levels in the fish’s blood.

The gills of a fish are made up of epithelium, which deliver deoxygenated blood by the arteries. Most fish have four gills on each side of their body.

The way that they filter water over the gills varies depending on the species of fish.

Some do this by widening their mouths to push water over the gills. Others, such as many shark species, push water over their gills by continuously swimming forward. This is where the idea that if sharks stop swimming they will die comes from.

Signs of Low Oxygen in the Water

If you’re reading this, you’re probably in the fishkeeping hobby. And that means you’re trying to learn more about keeping your fish healthy. One way of doing that is ensuring your fish has enough water in their fish tanks.

There are some specific signs to look for in your fish to recognize that there’s not enough oxygen in your aquarium.

  • Labored breathing
  • Rapid gill movement
  • Eating less than usual
  • Moving around less than normal
  • Spending more time around the surface of the water and gasping

If you notice any of these signs in your fish, they’re on the verge of suffocating and action must be taken quickly.

Immediately change up to 50% of the water to provide more oxygen in the water. After this emergency action has been taken, you’ll need to add some aerators, change filters or powerheads to something with a stronger motor (creating more air movement), and evaluate your stocking practices.

The most common cause of low oxygen levels is overcrowding in the fish tank. This means you’ll need to remove fish from your aquarium and give them a new home.

can fish drown purple yellow fish

What Causes Low Oxygen Levels?

There are multiple causes of low oxygen levels in an aquarium. As mentioned above, however, the most common cause is overcrowding of the fish tank. This means too many fish in too small a space.

Just like in a submarine, spaceship, or other air tight space there is limited oxygen for humans to share, a fish tank has only so much oxygen to go around.

Other causes for low oxygen levels include:

  • Live plants, especially in dark tanks
  • Low light levels
  • Elevated water temperatures
  • Certain chemicals
  • Excess waste
  • Algae blooms
  • Decomposing plants
  • Slow or zero water movement

Can There Be Too Much Oxygen in the Water?

Surprisingly, yes, there can be too much oxygen in a fish tank, as well. Excessive amounts can cause death in fish as well. They must have a balance of oxygen and hydrogen in their water to survive.

How To Improve Oxygen Levels in an Aquarium

To combat the various causes of low oxygen, you’ve got several options, starting with dealing with the most common issue – overstocked aquariums.

1. Keep the Right Amount of Fish Stocked

Every single fish you introduce into your aquarium effects the oxygen levels. Study your fish species and the maximum per gallon. Be sure to aim lower that than number and definitely go with the lean side.

2. Increase the Aquarium Surface Area

Another important aspect to keeping your aquarium properly oxygenated is making sure that there is enough surface area for the gas exchange that takes place on the surface of the water. Instead of using small or tall aquariums, go with long shapes like traditional rectangles, bow fronts, or the long, low aquariums.

3. Boost Water Movement

Invest in some spray bars, aquarium bubblers, powerheads, or wavemakers to create some aerating water movement inside the aquarium. Be sure to study the water flow needed for your particular species, however, to avoid over-doing it on the flow.

4. Keep the Right Temperature in the Aquarium

Finally, keep your aquarium to the right temperature. Cooler water naturally contains more oxygen. Be sure to shield your aquarium from sunshine and go with the lower end of the temperature range for your fish. And invest in fans to cool the air or an aquarium chiller.

Do Fish Drown If They Are Pulled Backwards?

Knowing what you now know about fish, gills, and all the amazing wonder that fish are, you could be wondering can fish suffocate if they are pulled backwards through the water.

The simple answer is yes, they can. The reason they can is because the water is flowing in the opposite direction and the fish will not be able to absorb the needed oxygen. Their gills can’t gain access this way as easily.

Some fish may be able to process the oxygen as they are pulled backwards, but most often, they become over-exhausted from the effort and wind up drowning because of not being able to keep up with the effort.

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