Why Can’t Saltwater Fish Live In Freshwater?

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Before you can set up an aquarium, it is essential that you first determine what type it is going to be. Will you go for the freshwater type or the saltwater variant? 

The type of fish and set-up will then follow depending on your choice. Each fish type can thrive in specific environments with certain other fish types and particular food. Disturbing this natural balance would result in unhappy and unhealthy fishes and a fish tank environment that nobody wants. 

Not only that but also living in adverse environmental conditions might even cause your fishes some problems or worse, death. If you have ever wonder why can’t saltwater fish live in freshwater and vice versa, this article can help you out.

Before delving into the explanation, it is essential to discuss these key concepts to understand the topic better. 

What is Osmosis? 

Osmosis is the movement of water molecules from a low-concentrated solution to a higher-concentrated one via a semipermeable membrane. This membrane is described as semipermeable since it only allows smaller molecules such as water to pass through while it simultaneously blocks bigger ones like salt and sugar. 

To illustrate, a raisin swells up when you submerge it in freshwater. This size increase results from the water flowing in the raisin due to its high sugar concentration. On the other hand, the raisin shrivels up when placed in saltwater as water flows out from the raisin due to the water’s high salt concentration. 

What is Osmotic Pressure? 

Osmotic pressure is the pressure applied to offset the flow of water across the semipermeable membrane. 

This pressure increases whenever water molecules move across the membrane to go to the higher concentrated solution. If both solutions have the same levels of concentration, then there is no need for movement and, thus, no osmotic pressure. 

What is tonicity and its different classifications? 

The measure of osmotic pressure is referred to as tonicity, and it has three classifications: 

  • Hypertonicity 
  • Isotonicity 
  • Hypotonicity 

To understand why fishes can only survive in specific environments, it is vital to know more about this. 


Hypertonicity occurs when there is a higher content concentration outside of the cell. As a result, the object shrinks since its water molecules flow out of it to dilute the outside environment’s concentration levels. 


Hypotonicity is the opposite, wherein the object contains a higher concentration level compared to its environment. As such, water molecules flow inside the cell to dilute the concentration, causing it to bloat and burst. 


Isotonicity occurs when both the content concentration levels of the cell and its outside environment are the same. As such, there is no need for water movement. 

saltwater fish

Why does salt concentration matter? 

Not all fishes can handle all levels of salinity, and their survival depends on the amount of saltiness their body can sustain. As such, they need to maintain a careful balance of water and salt concentrations to survive. This is done through a process called osmoregulation, which is basically the regulation of osmosis. 

Salt concentration matters as it determines the type of solution the water has. This, in turn, helps determine the type of fish that can survive and thrive in those conditions. 

How do fish deal with the salt concentrations in the water? 

Through osmoregulation, fishes can maintain balanced levels of both water and salt levels within their bodies. 

Fishes osmoregulate using their gills, intestines, kidneys, and other body parts. These organs are responsible for absorbing and excreting water molecules to help fish adapt to the water’s salinity levels. 

Seawater is basically a hypertonic solution such that it contains high salt concentrations compared to that inside the fish. As a result, the water inside the fish tends to flow out to dilute this solution. 

To compensate for the water loss, saltwater fishes continuously drink lots of water while excreting the excess salts and urea. 

In fact, most marine fish, sea turtles, and some reptile types have a salt gland that actively extracts chloride and sodium from their blood levels and excretes them as a concentrated solution. Moreover, the gills in saltwater fish also produce ion channels to help excrete excess sodium. 

On the other hand, freshwater is hypotonic to the fishes surrounding it. Since the fish’s body has a higher salt concentration, water molecules from the outside environment continuously flow into them to dilute the concentration. To prevent bloating, freshwater fish tend to urinate often. 

What are the different types of fish that can live in specific environments? 

Different fish species handle salt concentrations differently. Here are the different types: 

Stenohaline Fishes 

Stenohaline fishes are those species that prefer and stick to salt concentration levels that their bodies have already adapted to. Waters with salt concentrations of more than 0.05% would be fatal to most of these freshwater fishes. 

An example would be goldfishes, which are very sensitive in terms of environmental changes. Their bodies can only adapt to a particular environment, and if there are changes, they tend to react adversely to it. 

Euryhaline Fishes 

Euryhaline fishes are fish types that can adapt well to constant and rapid salt concentration fluctuations. They are fascinating creatures as once they detect changes in the water’s salt levels, their bodies then switch between absorption or excretion to adapt. 

With their make-up, they can quickly move back and forth between bodies of saltwater and freshwater.

Euryhaline fish types, the anadromous and the catadromous. 

On the one hand, anadromous euryhaline fishes are those originally born in freshwaters. However, they spend most of their lives in saltwater as they only return to their freshwater habitats to breed and lay eggs. Some examples of these include salmon, shad, smelt, striped bass, sturgeon, and trout. 

On the other hand, catadromous fishes live and spend most of their lives in freshwater and only go to the ocean to breed and lay eggs. An example would be the European eel and Mollies.


The third type of fish osmoregulates by matching its body’s salt concentration levels with those of its surroundings. As such, these fishes retain and synthesize salts and urea instead of excreting them. 

Since salt concentration levels from both sides are identical, no osmotic pressure takes place. A famous example of an osmoconformer would be sharks. 


What happens if you put a saltwater fish in a freshwater tank and vice versa? 

Saltwater fishes have a significant level of salt concentrations in their bodies. On the other hand, freshwater has little to no salt molecules in it. 

As such, the freshwater solution would be hypotonic to the saltwater fish such that water molecules would continuously flow inside the fish to dilute the salt molecules. However, it will have a hard time replenishing its salt and urea deposits. As it keeps happening, the fish will be bloated and then later explode. 

On the other hand, putting a freshwater fish in a saltwater tank would also be fatal. Saltwater has a higher salt concentration than the freshwater fish’s internal deposits, making it a hypertonic solution. 

As such, the water inside the fish would then dilute the saline concentration in the environment. This causes the fish to shrivel up and die since their bodies were not able to keep up with the excess salt deposits. 

Can saltwater plants live in freshwater? 

Plants, just like fish, also contain cells that are suited to specific conditions. Freshwater plants certainly have genetic make-ups that respond well to freshwater habitats. Placing them in saltwater conditions might prove fatal as their cells are not used to it. 

Is a saltwater aquarium harder to maintain than a freshwater aquarium? 

The difference between the two fishtanks lies in the price of equipment. 

Generally, freshwater aquariums are cheaper and usually costs around $250 to $300 to set one up. 

For one, there are different fish varieties and food types you can buy, and they are also usually less expensive since there is an abundance. Maintaining a freshwater fishtank is even easier and more comfortable due to a wide range of cleaning supplies and equipment.

On the other hand, saltwater tanks usually cost around $600 to $650. Fishes and food types are generally more expensive since they are quite sensitive and limited in quantity. 

Cleaning and maintaining a saltwater fishtank is also a challenge as you’ll need to track and monitor the water levels meticulously. When cleaning, you’ll have to mix water and salt in a separate container before you can replace the water. 

Saltwater fish can also be harder to feed since they are used to hunting in the wild. As such, they might not take well to commercialized pellets and similar food. 


Not all fishes handle and react well to the different saline levels in their surrounding water. Generally, they can only take the conditions their bodies were made for.

As such, placing different fish species in their opposite environments would be fatal. However, if you have ever wondered why can’t saltwater fish live in freshwater and vice versa, hopefully, this article has provided you with enough information to answer your question. 


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Rita Wagenerhttp://thepetkeepers.com
Rita is a resident paw expert at Pet Keepers. A registered & licensed dog trainer, she also has a degree in animal nutrition, and runs her own dog training course.

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