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Did you know that putting fishes in the wrong aquarium can cause them to die in one of the worst possible ways? They may swell up and burst!

This tragedy can happen if you put your fish in a tank with the wrong kind of water.

You see, two varieties of aquariums exist – salt and freshwater. With the ideal kind of fish in them, you shouldn't have any issues.

So, how will we know fishes meant for freshwater or saltwater? What will happen if you put a saltwater/marine fish into a freshwater tank/aquarium?

We cover all of these questions in this article, including a FAQ section that addresses your possible questions about saltwater aquarium.

So, let's get started, shall we?

How Will Saltwater Fish React in Freshwater?

The internal composition of fishes matches prehistoric oceans' chemical environment, meaning they live in saltwater, freshwater, or both habitats. The amount of salt differentiates salt and freshwater, and this is also closely related to the fish species living in the water.

The process that describes how saltwater fish survives is called osmoregulation which interlays with other associated functions. 

We need to understand these processes before discussing how saltwater fish will respond in freshwater.

Now, let's put on your science hat and go through some definitions with us. (We know this sounds like high school all over again, ugh...)


Osmoregulation is a natural biological phenomenon by which organisms control osmosis's effects to protect their cells' health and keep themselves in peak condition.

Osmosis is a process by which liquid molecules move through a thin membrane wall of living cells. Another way to describe it is liquid molecules' motion from a low concentrated region/solute to another high concentration region across a semi or partially permeable wall/membrane.

You can find similarities between any semi-permeable wall/membrane and a wall barrier with tiny allowances that lets water through but don't allow other concentrated elements like sugar or salt.

Picture this:

If you put a raisin inside a tank with supersaturated salt water, it will become smaller as water moves out of it.

This happens because raisin fruits have a far smaller salt quantity than a supersaturated salt water solution. In another scenario, if you submerge raisin in a freshwater tank, it will expand because raisin has a higher sugar concentration than freshwater.

Osmotic Pressure

Osmotic pressure is the force/pressure that works against the water molecules' motion across a semi or partially permeable wall/membrane. Osmotic pressure will typically increase as the water molecules move through the membrane.

As it gets to the point where the solution content is the same on either side, there will be zero osmotic pressure because the water does not need to move through. Tonicity is how you measure osmotic pressure under three classifications:

  • Hypertonicity
  • Hypotonicity
  • Isotonicity

Let's check out what they mean.

  • Hypertonicity: This condition occurs when the concentration of content outside a cell is higher than within the cell. In this scenario, water molecules migrate away from the cell to change/dilute the exterior's content. This makes the cell shrink.
  • Hypotonicity: The opposite of hypertonicity - the concentration of the cell's content is larger than that of the external space. In this case, water molecules move into the cell from the outside. This will eventually dilute the cell solution as water molecules continue flowing inside, causing the cells to start swelling and eventually burst.
  • Isotonicity: In this case, the content concentration outside and inside the cell is the same, so there is no motion of water atoms/molecules, and the cell volume remains the same.

What Happens When Saltwater Fish is Put in Freshwater

Phew, now, all of that science stuff is gone. Let us get to the real stuff that you are dying to hear. 

Saltwater has higher salt concentrations than the fish residing in it, making it hypertonic (we discussed hypotonicity earlier, remember?).

This quality makes the bodies of fishes release water to their surrounding (tank water) through the process of osmosis explained above.

Saltwater fishes will therefore end up drinking a lot of salt water to make up for the moisture they lose to salty waters. They filter the excess salt through their gills and kidneys by urinating.

Now, for the main takeaway:

If you put saltwater fish in freshwater, the scenario it creates is hypotonicity because they have a much higher salt concentration than the freshwater around them. As explained above, the water molecules from the surrounding will keep flowing inside their bodies, and urination won't even be enough to keep it balanced. Eventually, the fishes may swell and burst, dying in the process.

Are There any Fishes that Survive in Salt and Fresh-Water?

Euryhaline fishes are a unique fish species that can live in freshwater as well as saltwater tanks. These fishes have physiological mechanisms that make them withstand both high and low salinity levels without difficulties. This makes them excellent in moving through salt and freshwater areas, including going from saline seas to freshwater rivers.

These fishes adapted to the different water environments because of the competitive pros of each water. It moves from freshwater to saltwater or conversely to get away from predators, unlivable temperatures, or look for food. 

As they move between both water conditions, there is an adjustment period where their bodies adapt to the change in salinity. They have a salty estuarine environment that helps them switch their salt balance physiology.

There are two main euryhaline fish species - anadromous and catadromous.

So, what's the deal with them?

Anadromous: These fishes spend a lot of their time in saltwater habitats but often come back to freshwater, where these fishes were born, to spawn. Even though they can live in the two water conditions, sudden increases or reduction in the salinity levels can be challenging.

Anadromous fish often associate different life stages with the different habitats' salinity levels, making it easier to regulate the salt content between the water surrounding them and their bodies. Examples of these fishes are striped bass, salmon, shad, smelt, and sturgeon.

Catadromous: These fishes, on the other hand, spend most of their time in freshwater habitats but move back to saltwater conditions to spawn. Examples are the North American and European eels.

Popular Saltwater Fishes 

Some of the most recognizable Saltwater fish are:

Striped Bass: This migratory species is found on the eastern US coast and is sleekier than many other common species, such as the White Bass.

Ocellaris Clownfish: This fish is most recognizable because of the Finding Nemo movie. It is an excellent fish for people new to fishkeeping. 

p/s: We do have a feeding guide on taking care of your Clownfish.

Bluefish: Bluefish is also a migratory fish you can find alongside the East Coast. These fish species are easily recognizable by their sharp teeth.

Lawnmower Blenny: This fish prefers to stay in the lower levels of your tank. They are herbivores that spend most of their time hiding in caves and only come out to find food. You might have to keep them singly in a saltwater tank as they tend to nip on fishes of a similar size.

King Salmon: The King salmon fish, also called Chinook, is among the most desired fish usually found close to the West Coast of the USA.

Redfish: You can find these fishes in areas around the Gulf of Mexico and the East Coast of the Atlantic. Among all of the saltwater fishes, people generally call the Redfish the high fighter of the bunch.

Pacific Halibut: As the name suggests, this is a Pacific flathead fish – the largest of them. They are also one of the most popular saltwater fishes, and it shouldn't be hard to find one of them.

Other saltwater fishes are:

  • Coral Beauty Angelfish
  • Royal Gramma
  • Chalk Bass
  • Six Line Wrasse
  • Firefish
  • Pajama Cardinalfish
  • Klein's Butterflyfish

Saltwater Fish Habitats

You can find saltwater fish in any of these three marine habitats

  • Pelagic Zone: This is a warm water column or migratory path that pelagic saltwater fishes swim through. This zone isn't near the bottom or shore of the water. Well-known pelagic fish examples are tuna and wahoo.
  • Demersal Zone: They are the lakes or seas situated near the lakebeds or sea floors. Demersal fish is any saltwater fish with a natural habitat around this zone. Common species of this habitat are the flounder and halibut.
  • Coral Reef Systems: This fish habitat has a lot of coral reefs. They are a widespread habitat for reef fishes like the very rare gag grouper and red snapper.


How Would a Freshwater Fish Respond to Being in Saltwater - can freshwater fish survive in saltwater? 

With the processes of hypertonicity, hypotonicity and isotonicity explained above, it will be easier to understand how freshwater fishes will fare in saltwater. 

To dilute the very saline surrounding water, water molecules from a freshwater fish will consistently move to the surrounding water.

The salt concentration in saltwater is too much for freshwater fish to drink, so there will be a lot of water loss, and eventually, dehydration will kill the freshwater fish.

Like saltwater fishes, freshwater fish also produces a lot of urine but with low salt content. These fishes live in low saline waters, meaning their kidneys don't have to work too hard to maintain their bodies' salt content.

Is a Saltwater Aquarium Good for Beginners? 

Saltwater fishes are harder to breed because they depend on seasonal cues to start spawning, and this may be challenging to recreate at home.

We have solid reasons to believe that beginners can still get a saltwater aquarium and have almost no problems maintaining it or taking care of the fishes. However, you have a lower allowance for error since saltwater fishes are far more sensitive to water quality.

A saltwater aquarium should not scare beginners as long as they take the time to research and get the appropriate saltwater tank. A great way to start is with a fish-only-with-live-rock (FOWLR) aquarium so you can gradually ease into it.

Can You Change a Saltwater Tank into a Freshwater?

You can change your saltwater tank into freshwater if you intend not to keep saltwater fishes again. Changing the water will not affect your fishes in any way if you are very careful with the process and make all the suitable adjustments to accommodate your freshwater fish.

How do you do this, you ask?

Drain the aquarium of any saltwater and also remove the decorations in the tank. Use chlorine-free water to wipe everything down in the tank and wash the decorations too. There must be no traces of salt in the aquarium that could harm freshwater fish.


There you have it! It is extremely dangerous to put a saltwater fish in a freshwater aquarium, except for the Euryhaline fish species.

Euryhaline fish species such as salmon are the only fishes with the unique ability to survive in salt and freshwater aquariums.

Even though saltwater aquariums need more care and experience, beginners shouldn't run away from the responsibility of taking care of saltwater aquariums. If it gets too overwhelming, you can change your saltwater tank into freshwater and switch the tank inhabitants.

The key to putting fishes in their right tank lies in the work and research you put into it. As long as you take the time to find out your fishes' ideal habitat, you should have no problem.

If you are still looking for more information on creating a lively aquarium, we cover all about it on our site.

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About the author 


My name is Eric. I'm the owner of snugaquarium.net and a writer with a passion for aquariums and fish-keeping. I love to watch the three different species of freshwater fish floating around in my homemade aquarium in my spare time.

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