DISCLOSURE: This post may contain affiliate links. As an Amazon Associate I receive a commission if you decide to purchase through my links. This at no extra cost to you.

How Would A Saltwater Fish Respond To Freshwater? Find Out Here!

how would a saltwater fish respond if it is put in an aquarium of fresh water

How Would A Saltwater Fish Respond If It Is Put In An Aquarium Of Fresh Water?

How would a saltwater fish respond if it is put in an aquarium of fresh water is the type of curiosity that every fishkeeper considers at one point or another. Saltwater tanks can be converted into freshwater tanks, so why wouldn’t fish be able to switch between the two?

The answer is complex (and we’ll break it down later on!), but it all boils down to salt concentration and how fish have developed physiological mechanisms to help them adapt to the environment they live in.

A fish’s ability to adapt to varying levels of water salinity (or lack thereof) usually determines whether or not it can migrate from saltwater to freshwater, or vice-versa.

Why Do Fish Typically Live Either In Saltwater Or Freshwater, But Not In Both?

will salt kill freshwater fish
Royal Gramma Basslet (Gramma loreto)

Fish live in either saltwater or freshwater, depending on which of the two have been the environment in which their fish species have traditionally survived in. Evolution allowed fish to develop physiological traits that make them compatible with either saltwater or freshwater.

The main difference between freshwater and saltwater is salt concentration.

This means that the distinction between freshwater fish and saltwater fish is the way they tackle the salt concentration in their respective environments.

Freshwater fish

Freshwater fish have physiological mechanisms in place that allow them to live in waters that have low salinity. By concentrating the salt deposits in their bodies, they counteract the low levels of salt in their natural environment.

To do this, freshwater fish will eliminate an impressive volume of extremely dilute urine throughout the day. They do this to get rid of excess water while retaining ions through their gills.

Saltwater fish

Saltwater fish have physiological mechanisms in place that help them excrete excess salt continuously as they drink marine water continuously.

There are enzyme-producing chloride cells present in the gills of saltwater fish. The enzyme naturally produced by a marine fish’s body is known as a sodium/potassium pump (Na+/K+ ATPase).

This enzyme will pump out sodium, flushing out excess salt through the gills. A saltwater fish’s kidneys also aid in filtering out salinity-regulating ions.

Euryhaline fish

Euryhaline fish are the fish species that have retained the salinity-regulating physiological mechanisms of both saltwater fish and freshwater fish.

They can survive in both environments.

The physiological mechanisms that dictate whether a fish can survive in saltwater or freshwater, or in both, work through a process called osmosis.

What Is Osmosis And How Does It Work?

what would happen if a saltwater fish was placed in freshwater
Ocellaris clownfish (Amphiprion ocellaris)

Osmosis is the process through which water molecules will diffuse across a permeable membrane, from a high-concentration area to a low-concentration area.

It sounds more complicated than it is!

Osmosis basically allows water to move into and out of the cells of an organism.

Fish cells have semi-permeable membranes. Through osmosis, the concentration of salt inside a fish’s body and the salinity of the water it lives in will eventually even out.

The process of osmosis can be harmful (even deadly) to any living organism.

Think of it this way: what happens when you put a piece of dehydrated fruit in a container of freshwater?

It soaks up the water continuously. That happens because the sugar concentration inside the dehydrated fruit is higher than the sugar concentration in the freshwater around it.

If you were to put a piece of dehydrated fruit in a container of saltwater, it would shrink (and dehydrate) further.

Because saltwater has a higher concentration of salt than the dehydrated fruit has, the water in the fruit will flow out continuously.

The factors that dictate the in-flow vs. out-flow of water into an organism through osmosis are osmotic pressure and osmoregulation.

Osmotic Pressure | Osmoregulation

How long can a saltwater fish live in freshwater
Powder Brown Tang (Acanthurus japonicus)

Osmoregulation is the active process that regulates the osmotic pressure of an organism’s water content.

Osmoregulation in fish has the purpose of maintaining an internal balance of salt and water inside a fish’s body, also known as osmotic balance.

The semi-permeable skin of fish, especially around their gills, is subjected to constant osmotic pressure.

Freshwater fish have a higher osmotic pressure than the water they live in, while saltwater fish have a lower osmotic pressure than seawater.

Osmotic pressure

Osmotic pressure dictates the direction in which osmosis will work. Taking it back to the dehydrated fruit example, osmotic pressure will dictate whether water will flow in or flow out based on the tonicity of the dehydrated fruit vs. the tonicity of the type of water it’s placed in.


Tonicity is how we measure osmotic pressure. The three levels of tonicity are the determining factors of why freshwater fish can only survive in freshwater, and saltwater fish can only survive in high-salinity water.

The three levels of tonicity are:

  • Hypertonicity

Hypertonicity is a level of osmotic pressure that occurs when the content concentration outside of a cell is higher than inside of it.

When a hypertonic organism is placed in a hypotonic body of water, its cells will shrink as water flows out in an effort to dilute the water outside to achieve osmotic balance. 

  • Isotonicity

Isotonicity is a form of osmotic balance that requires no exchanges between a cell’s interior and its exterior through a permeable membrane.

For example, red blood cells will be isotonic in a plasma solution.

  • Hypotonicity

Hypotonicity is a level of osmotic pressure that occurs when the inside of a cell has a higher content concentration than the outside.

When a hypotonic organism is placed in a hypertonic body of water, its cells will swell up as water flows in, diffusing the cell’s salt concentration to achieve osmotic balance. 

So, how does tonicity determine whether a saltwater fish can live in freshwater, or if a freshwater fish can live in saltwater?

You have to take into account the tonicity of both bodies of water and both types of fish.

Due to their distinct tonicity levels, osmoregulation in fish can be deadly if you were to place them in the wrong body of water.

Osmoregulation | Freshwater Fish Vs. Saltwater Fish

how long can a saltwater fish live in freshwater
Mandarinfish (Synchiropus splendidus)

In a perpetual state of osmoregulation, freshwater fish and saltwater fish are both adapted to take on or release water from their surrounding environment to maintain an osmotic balance.

Here’s a side-by-side view of how osmoregulation in freshwater fish vs. saltwater fish works.

Freshwater fish

Saltwater fish

Freshwater fish are hypertonicSaltwater fish are hypotonic
Freshwater is hypotonicMarine water is hypertonic
Freshwater fish are constantly taking in water.Saltwater fish are constantly releasing water out.

This means that freshwater fish face a constant influx of water while losing salt because of water diffusion.

Saltwater fish, on the other hand, are in a constant battle with dehydration and an excessive influx of salt.

If you were to put a fish into the wrong type of water (saltwater fish in fresh water / freshwater fish in salt water) osmoregulation would be the “cause of death” on that fish’s death certificate.

But there are exceptions to this “rule”.

What Happens If You Put Saltwater Fish In Freshwater?

why do fish live in saltwater
Pajama cardinalfish (Sphaeramia nematoptera)

Saltwater is hypertonic to saltwater fish, so water is constantly being drawn out of a fish’s body.

To survive and maintain osmotic balance, saltwater fish will drink copious amounts of seawater to compensate for the constant loss of water.

Excess salt is also excreted, either through their gills via the bloodstream or by passing through their kidneys and being eliminated as urine.

Being acclimated to this osmoregulation process, a saltwater fish would die if put in a tank of fresh water and left there.

Freshwater dips for saltwater fish are totally different from attempting to “convert” a fish from being saltwater-adapted to freshwater-adapted. More on that later!

Why would a saltwater fish die if put in an aquarium of fresh water?

A saltwater fish will obviously have a high concentration of salt itself. If put in a tank of fresh water, a saltwater fish’s cells would swell continuously, trying to dilute its salt concentration.

The effects of osmosis are deadly for saltwater fish when placed in freshwater. As a fish’s cells begin to swell, its body will take in excess water, causing a condition called osmotic lysis.

Osmotic lysis causes cells to simply burst when an osmotic imbalance causes excess water to enter the cells of an organism (i.e., fresh water constantly flowing into the cells of a saltwater fish).

How long can saltwater fish stay in fresh water before they risk dying?

Fishkeepers that have experience with setting up and maintaining a reef tank are definitely familiar with the concept of a “freshwater dip” for saltwater fish.

Aquarists use this shock-method when marine fish develop a condition known as the “ich” or white spot disease. It is a parasitic infection that most saltwater fish are vulnerable to.

A freshwater dip is meant to kill the parasite, taking advantage of osmosis.

The parasite itself, being no larger than 1mm in its adult form, will succumb to the negative effects of osmosis (saltwater organism in freshwater) much quicker than it would take for the “freshwater dip” to harm the diseased saltwater fish.

The parasite’s cells will burst in an instant as excess water flows into its cells. That’s why fishkeepers that use the “freshwater dip” to treat the “ich” say that even a 30-second dip will do the trick.

How do saltwater fish drink water?

Saltwater fish have a lower salinity level than seawater does, so water is constantly getting drawn out of their cells. This means that marine fish have to drink water to stay hydrated.

Unlike freshwater fish, saltwater fish will pass some of their water intakes through their digestive tract/kidneys while also excreting excess water through their gills.

To get rid of the excess salt that inadvertently comes with drinking seawater, saltwater fish use the chloride cells in their gills.

Are there any saltwater fish that don’t need osmosis to survive?

Saltwater fish that don’t rely on osmosis to achieve osmotic balance are called osmoconformers.

The blood of osmoconformers is isotonic to the body of water they live in.

This means that the concentration of salt in an osmoconformer’s body is equal to the salinity of seawater, making osmosis unnecessary, as the osmotic balance is maintained naturally.

Sharks, for example, don’t need osmoregulation to survive. They do this by storing the urea that most organisms produce and excrete as urine.

Storing urea gives shark cells the ability to maintain the same salt concentration as the seawater they live in.

Sharks also have a rectal gland that secretes salt, which assists their bodies in staying in osmotic balance.

What Happens If You Put Freshwater Fish In Saltwater?

Blue green chromis (Chromis viridis)
Blue-green chromis (Chromis viridis)

Some fish can tolerate only the smallest fluctuations in water salinity. These fish are known as stenohaline fish.

Freshwater stenohaline fish will not survive in bodies of water that have a salinity level higher than 0.05%.

Goldfish are the most popular freshwater aquarium fish that are stenohaline.

On the other hand, there are marine fish species that are stenohaline and can only survive in high salinity water, such as the haddock fish.

If you were to put a freshwater fish into a saltwater tank, it would die of dehydration.

Freshwater fish can simply not survive in seawater because of the high concentration of salt.

Through osmosis, the water inside a freshwater fish’s cells would be drawn out, and it would get fatally dehydrated very quickly.

How do freshwater fish drink water?

Freshwater fish don’t actually drink water to hydrate themselves.

Due to the fact that freshwater is hypotonic to their bodies, water will constantly flow into the cells of a freshwater fish. The influx of water comes through their skin, their gills, and through their mouths as they feed.

To get rid of the excess water, freshwater fish will eliminate abundant amounts of urine, sometimes even up to a third of their weight per day.

How Can Some Fish Live Both In Saltwater And Freshwater?

saltwater vs freshwater fish
Silverspot Squirrelfish (Sargocentron caudimaculatum)

Fish species that can survive in both saltwater and freshwater are called euryhaline fish.

True euryhaline species are those fish species that will not only survive in both seawater and freshwater but will also be able to breed in both types of bodies of water.

Atlantic stingrays and Largetooth Sawfish Pristis are the only marine euryhaline fish species recorded to be able to reproduce in both saltwater and freshwater environments. 

Euryhaline fish species that will find it easiest to migrate from an ocean into a river, and vice versa, are those that can withstand large variations in salinity levels.

The most adaptable euryhaline fish species are:

  • Common Molly;
  • Flounder;
  • Channel Bass;
  • Pink Salmon;
  • Green Crabs (euryhaline invertebrate);

A fish species’ tolerance to salinity is the deciding factor of whether it can survive in both freshwater and saltwater. Depending on what concentration of salt a fish’s body can tolerate, there are two main types of euryhaline fish:

Anadromous fish

Anadromous euryhaline fish are born in freshwater environments and will only return to their native location to breed. They spend the majority of their lifetime in seawater.

Some popular anadromous fish species are:

  • Salmon;
  • Shads;
  • Smelt;
  • Sturgeon;
  • Striped bass.

Catadromous fish

Catadromous euryhaline fish are born in saltwater, but they will migrate to bodies of fresh water as juveniles. There they will spend the majority of their life choosing to go back into the ocean only to spawn.

Popular catadromous fish species:

  • European eels;
  • American eels;
  • Longfin eels;
  • Shortfin eels;
  • Inanga fish.

Just because euryhaline fish species can live in both saltwater and freshwater doesn’t mean that they don’t need to go through an acclimation process (osmoregulation) to adjust their bodies to the varying levels of salinity.

Both anadromous fish and catadromous fish only go through this type of migration at certain points of their lives (as juveniles/when reaching sexual maturity).

So, achieving osmotic balance in such a wide range of salinity levels isn’t a physiological process they have to go through too often.


How would a saltwater fish respond if it is put in an aquarium of fresh water?

The answer depends on how long you keep said fish in a body of water that it’s not evolutionary-equipped to survive in.

A dip in fresh water might rid your saltwater pet fish of the “ich”, but a longer stay in a freshwater tank will 100% kill it.

Osmosis and osmoregulation are the science-based biophysiological processes that prove this to be true, so it’s nothing that a fishkeeper should experiment with because pet fish will be lost in the process.

Fun fact: Most animals that spend their lifetimes in a marine environment have particular evolutionary traits that help them eliminate excess salt.

Sea otters will eliminate highly-concentrated urine, turtles can cry salty tears, and seabirds will use salt glands to process sodium and eliminate it as a salty secretion through a duct on the tip of their beaks.

Here’s a quick video explaining how saltwater fish drink water vs. how freshwater fish do it:

Recommended Reading: