do fish drink water?

Do Fish Drink Water? Do Fish Ever Get Thirsty?

Have you ever wondered that creatures living underwater need water to fulfill their thirst? Do fish ever get thirsty? Do fish drink water?

So, to answer the question we need to understand that why we drink water? The answer is that water is necessary for carrying out vital chemical reactions. Water is a universal solvent and without it, we will die in no time.

Similar chemical reactions are taking place in fishes as well. So, it becomes clear that fishes do need water as well, but do they need to drink it to carry out these reactions.

The answer is “it depends on their habitat, Freshwater fishes do not drink water, they just absorb it from their body surface. Saltwater fishes do drink plenty of water and excrete a small volume of concentrated urine”.

Osmosis is the main reason behind this behavior shown by fishes according to the habitat.

Let’s discuss this phenomenon in detail and find some answers.

What is osmosis?

Biologically speaking, Osmosis is the movement of water from an area of higher solute concentration to lower solute concentration through a semi-permeable membrane.

This phenomenon is pretty common in our daily life. Such as in plants the movement of nutrients from roots to the stem, leaves, and fruit is all thanks to osmosis.

How does osmosis affect fish’s behavior?

As described above osmosis plays a significant role in fish’s life as well. The mechanism is simple but quite fascinating, it all comes down to the concentration of salt in water as compared to that in a fish’s body.

The concentration of salt in freshwater is quite low as compared to that in the fish’s system. Thus, there should be a net movement of water inside the fish body due to osmosis. Exactly same mechanism occurs in saltwater but the opposite direction from the fish’s system to the water.

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Why freshwater fish Do not drink water?

A fish living in a freshwater environment is surrounded by decreased salt concentration. This means the blood and bodily fluids of a freshwater fish contain higher volumes of s This means the blood and bodily.

Thus, a concentration gradient is formed, and water moves inside the body through the body’s surfaces.

So, a freshwater fish does not need to drink water and thus a freshwater is never thirsty.

In addition, fish living in freshwater conditions produces a large volume of dilute urine. This urine helps eliminate unnecessary salts in the body without putting too much stress on fish.

How do freshwater fish absorb required water?

As we figured it earlier freshwater fish excretes a large volume of dilute urine, so a question arises how do these fishes get enough water from when they do not even drink it at all?

Well, freshwater fish absorbs most of the water from their bodies. Moreover, as the water flows through the gills most of it is absorbed and becomes a part of their system. This inflow of water results in diluting their blood and successfully carrying out major biochemical reactions.

How does freshwater fish deal with the constant inflow of water?

Although freshwater fish do not drink water at all, still their body is constantly replenished with plenty of water. This excess fluid if not treated carefully will result in swelling of cells and tissues of freshwater fish.

This problem is solved by two main mechanisms.

  1. A large volume of urine:

Firstly, excess water is removed through the production of a large volume of urine. Plus, salt is conserved through the formation of dilute urine.

  • Active salt pumps in gills:

Interestingly, freshwater fish have active pumps that constantly pump in the salt against its concentration gradient.

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This inward pumping of salt is responsible for the conservation of salt in the body.

Why do saltwater fish drink plenty of water?

From our above discussion, the answer to this question becomes quite obvious. Saltwater has a higher concentration of salt than that in fish’s blood. This helps in developing a concentration gradient resulting in the outflow of water according to osmosis.

 As water is constantly being removed from the fish’s surface, thus, saltwater fish needs to consume bulks of water daily. Thus, a saltwater fish drinks a lot and is almost always thirsty.  

Moreover, to conserve this water, this is commonly seen in saltwater fish that produce a very small amount of concentrated urine. The concentrated urine is eliminated to remove excess salt taken in during drinking plenty of saltwater.

How does a saltwater fish gets rid of this excess salt?

After drinking hordes of saltwater, a saltwater fish needs to eradicate this excess salt, this is achieved through the following mechanisms.

  • Production of concentrated urine:

Well, to remove excess salt taken in during drinking saltwater, fish’s kidneys are evolved to produce a small concentration of highly concentrated urine. This way fish can conserve water as well as lots of salt is also excreted.

  • Active salt pumps in gills:

In saltwater fish, there are special pumps located in their gills. These pumps work against the concentration gradient and actively pumps salt out of fish’s blood into the saltwater.

This way a minimal but sufficient concentration of salt is maintained in the fish’s body.

How does a saltwater fish prevent dehydration?

It is obvious from the study of osmosis that a saltwater fish is constantly losing water from its surface, exactly opposite to that occurring in freshwater fish.

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This water is lost due to the concentration difference between fish’s body fluids and surrounding water. Ultimately saltwater fishes are always dehydrated and are at risk of having their skin shrink up.

This problem is countered by drinking tons of water and excreting just a little of it. That’s all for saltwater fish and that is their key to survival.

How do sharks differ from freshwater and saltwater fish?

Sharks are quite interesting creatures, but their mechanism of drinking water is quite impressive. Sharks conserve a large number of salts in their bodies, their blood staying at a higher salt concentration than saltwater.

Thus, according to osmosis, they do not need to drink water at all. Sharks are constantly absorbing water from their surfaces due to diffusion. So, SHARKS DO NOT DRINK WATER JUST LIKE A FRESHWATER FISH BUT IN A SALTWATER ENVIRONMENT, ISN’T IT INTERESTING.

Sharks have plenty of salt pumps located in their gills as well as the rectum that help in pumping out excess salts from their body.

Thus, for sharks, the key is the conservation of waste such as urea resulting in rendering them saltier than seawater, and ultimately, they do not get thirsty at all.


In conclusion, osmosis is the key to all the biological and chemical processes happening in a fish’s body. Moreover, a fish’s habits of drinking water are all based on their surrounding environment.

For a freshwater fish, there is no need to drink water at all just diffusion of water through the body surface is enough.

In comparison, saltwater fish drinks a load of water every day to keep up their biochemical reaction and to prevent dehydration.

We hope this article will be beneficial and will answer all your question.

Best of luck with your aquarium!!!

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