Why can’t saltwater fish live in freshwater?

In this blog post, we will answer the question “Why can’t saltwater fish live in freshwater?” and learn more about why freshwater fish can not also live in saltwater. We will see explanations of osmotic processes and the general physiology of fish and their habitats.

Why can’t saltwater fish live in freshwater?

Saltwater fish cannot live in freshwater because they would die. This means that the fish’s cells would break down. Saltwater fish in freshwater are saltier relative to their environment, so they cannot survive because the surrounding water would enter the cells and they would expand until they broke apart and died.

Fish are very sensitive to changes in salinity. To set up an ideal aquarium, it is important to know the physiology and key characteristics of your fish. Saltwater fish require greater care with water quality, as it directly affects the bodily functions of your fish.

The impossibility of saltwater fish living in freshwater is closely related to the physiology of fish. In other words, it is about how the fish body functions, gas exchange, and the exchange of nutrients and water with the environment. We will also have a look at the processes of tonicity and osmoregulation.

This topic is about tonicity and osmoregulation, physiological functions of the fish body. These processes represent a major evolutionary step in the history of life on Earth. They also enabled the exploration of the largest habitat on our planet, water. 


The reason why freshwater fish cannot survive in saltwater and vice versa has a lot to do with the property of any liquid called tonicity. Simply put, it is the capacity of a solution with osmotic pressure across a membrane.

Tonicity can classify structures and environments as:

– Hypertonic;

– Hypotonic;

– Isotonic.


Hypertonic means that the concentration of a solute in one solution is greater than the concentration of another solution. It must always be remembered that the solutions are separated by a semipermeable membrane. When the environment is hypertonic and a cell is immersed in it, water moves by osmosis from the less concentrated medium to the more concentrated one, in this case from the inside of the cell to the outside.


A hypotonic solution has a lower solute concentration than another solution, and the two are also separated by a semipermeable membrane.

By osmosis, the solvent, usually water, tends to move from the hypotonic solution to the hypertonic environment.


An isotonic state exists when a structure and its environment are in isotonicity. This means that the rate of substances entering equals the rate of substances leaving.


Osmoregulation can be defined as the ability of some organisms to control the concentrations of salts or solutes in tissues to maintain the regularity of metabolic activities necessary for their functioning. This regulation is essential for the survival of animals.

Take, for example, a fish in a hypertonic environment. It would lose excessive water by osmosis, which would lead to its death. The opposite would happen to a fish in a hypotonic environment, as it could take in water incessantly.

Strategies of osmoregulation

Since there are different aquatic environments with different salt concentrations, such as freshwater and saltwater, different strategies are required to overcome any physiological problems associated with salt and solvent concentrations. These strategies of regulation with their respective fish groups are explained in the following subsections:

– Osmoregulation in freshwater bony fish;

– Osmoregulation in marine bony fish;

– Cartilaginous fish.

Osmoregulation in freshwater bony fish.

Freshwater fish are in a hypotonic environment with respect to their internal body structures. This means that the concentration of solutes inside the fish body is higher than the concentration in the external environment. If there were no osmoregulatory mechanisms, the fish would take in water until the concentrations were balanced, which could lead to the death of the fish.

The mechanisms present in these fish ensure that the entry of water does not affect their physiological functions. In freshwater fish, water enters through the gills, as do the necessary solutes. However, the entry of water is compensated by the excretion of this substance through the very dilute urine produced in very efficient kidneys.

Osmoregulation in marine bony fish

Saltwater fish live in an environment that has a much higher concentration of salt than their bodies. If there were no osmoregulation mechanisms, the fish would lose a large amount of water, which would affect its survival.

To keep the amount of water and salts in their bodies in balance, saltwater fish take in a significant amount of seawater and effectively excrete the excess salt. This excretion occurs through cells in their gills that eliminate excess salt through active transport. The urine of marine fish, unlike the urine of freshwater fish, is very concentrated so that they do not lose more water.

Cartilaginous Fish

Cartilaginous fish live in the marine environment and have a large amount of urea in their blood. This amount of urea keeps the solute concentration in the body similar to the solute concentration in the environment. As the concentrations become isotonic, there is no water loss or gain.

But do fish drink water?

Freshwater fish do not actively drink water through their mouths because if they do, there is a risk of diluting their blood. Saltwater fish, on the other hand, drink a lot of water to stay hydrated at all times. As we have seen in this article, freshwater fish tend to absorb water into their bodies, while saltwater fish lose a lot of water to the environment in which they live.


In this post, we have explained why you cannot keep saltwater fish in freshwater aquariums and vice versa. We have also described some topics to understand the physiology of different fish species and have learned about the chemical and physiological processes of these organisms.

Frequently Asked Questions (FAQs): Why can’t saltwater fish live in freshwater?

What about fish that live in estuaries?

Some fish species can live in both environments, freshwater and saltwater. These species are called euryhaline fish. However, most fish species can only survive in one environment or the other. The characteristics that determine which environment is ideal are their salt tolerance, or in other words, how much salt their bodies can tolerate.

Do fish get thirsty?

Freshwater fish probably do not feel thirsty. This is because, in a hypotonic environment, they tend to absorb water from the environment through osmosis via their gills. So they do not need to drink water and probably do not feel thirsty. Saltwater bonefish, on the other hand, need to drink water from the environment all the time because as soon as they are in a hypertonic environment, they lose water. So they are the ones most likely to feel thirst.

Can you convert saltwater fish to freshwater?

The answer is YES, you can convert saltwater to freshwater. Desalination is a physicochemical treatment process that removes excess mineral salts, microorganisms, and other solid particles from salt and brackish water to produce freshwater. This system can directly lead to cheaper water filtration. However, if you have decided to switch from your saltwater aquarium to a freshwater aquarium, you need to think about the organisms you already have. If they are not euryhaline species, you will need to find a new home for your fish, corals and plants. Saltwater plants cannot survive in a freshwater aquarium, nor can saltwater fish.

Which is easier, saltwater or a freshwater aquarium?

Maintaining a saltwater aquarium is not as easy as maintaining a freshwater aquarium. Saltwater aquariums are more expensive and require more maintenance. Many people choose to have a saltwater aquarium simply because it looks much nicer. You can keep colourful corals in saltwater aquariums. These animals add colour and fantastic shapes to the environment. Freshwater aquariums are much easier to maintain and less expensive than saltwater aquariums. Even though some people think that freshwater fish are not colourful, they are completely wrong. There are goldfish, tetras, betta fish, guppies and others.

How do fish drink water?

Saltwater bonefish drink more water than freshwater fish. This is because the hypertonic environment draws water from the bodies of saltwater fish. So, to maintain their fluid balance, they must take in water from the environment. Water intake is similar to the way food is ingested. The water is taken in through the mouth, the fish swallows it and it enters the digestive tract.

Do fish pee?

Yes. But not every fish pees in the same way. Freshwater fish must excrete excess water that accumulates in their bodies by osmosis, which causes water to enter their bodies. Their kidneys produce a lot of urine to prevent their tissues from becoming waterlogged. In contrast, saltwater fish, which already lose water through osmosis, produce much less and very concentrated urine.


Kültz, D. (2015). Physiological mechanisms used by fish to cope with salinity stress. The Journal of Experimental Biology, 218(12), 1907-1914. 

Cao, Q., Gu, J., Wang, D., Liang, F., Zhang, H., Li, X., & Yin, S. (2018). Physiological mechanism of osmoregulatory adaptation in anguillid eels. Fish physiology and biochemistry, 44(2), 423-433.

Tipsmark, C. K., Sørensen, K. J., & Madsen, S. S. (2010). Aquaporin expression dynamics in osmoregulatory tissues of Atlantic salmon during smoltification and seawater acclimation. Journal of Experimental Biology, 213(3), 368-379.

Alderman, S. L., Dilkumar, C. M., Avey, S. R., Farrell, A. P., Kennedy, C. J., & Gillis, T. E. (2020). Effects of diluted bitumen exposure and recovery on the seawater acclimation response of Atlantic salmon smolts. Aquatic Toxicology, 221, 105419.