How Would a Saltwater Fish Respond in a Freshwater Aquarium?

You might be tempted to believe that all fish are identical in terms of survival, meaning they will survive in all types of water such as in a river, pond, lake, and sea. After all, they need water for survival.

However, this is far from true.

It turns out that there are two types of water namely, freshwater and saltwater. Some fish live in only one type of water, while others can survive in both.

While we discuss this in detail in this article, the short answer is that a saltwater fish most likely won’t survive in freshwater because of high salt content in their body. However, there are certain fishes that can live in both salt and freshwater aquariums.

How Would a Saltwater Fish Respond in a Freshwater Aquarium?

Ever since the marine life began evolving billions of years ago, fish have maintained an internal composition matching the ionic environment of those primeval oceans. Today, fish live in either freshwater or saltwater or both.

The most obvious difference between the two water environments is the salt content. The salinity of the surrounding waters has a lot to do with the fish living in them.

If we specifically discuss the saltwater fish, it cannot survive in a freshwater aquarium. To understand this phenomenon, it is important to understand osmoregulation and some related processes. It is a biological process through which living things regulate the impact of osmosis to safeguard the health of their cells.

Osmosis occurs when liquid molecules pass through a wall called membrane in a living cell. More specifically, it is the movement of liquid molecules from low concentrated solute to a high concentrate solute through a semi-permeable membrane.

A semi-permeable membrane is a wall-like barrier consisting of small holes that allow water to pass through but block other concentrated content such as salt, sugar, etc.

For instance, if you submerge raisin in a supersaturated saltwater aquarium, it will shrink as the water will flow out because raisin has lower salt content than the supersaturated saltwater aquarium. On the other hand, if you put a raisin in freshwater, it will swell up since raisin has higher sugar content than the freshwater.

There’s another term called osmotic pressure, which is the pressure applied against the movement of water molecules through a semi-permeable membrane.

The osmotic pressure increases as water molecules flow across the membrane. When the solution is equal on both sides of the membrane, no osmotic pressure is applied because there’s no need for the water to pass through.

As we move closer to answering the original question of why saltwater fish cannot survive in a freshwater aquarium, let’s also understand the measure of osmotic pressure referred to as tonicity.

There are three classifications of tonicity namely hypertonicity, hypotonicity, and isotonicity, each of which has to do with cells in living organisms.

Hypertonicity is a condition in which the content concentration is higher outside the cell than inside of it. Water molecules move out of the cell in this case to dilute the content outside.

As a result, the cell shrinks. Hypotonicity is the complete opposite: The content concentration is higher inside the cell than outside of it. Water molecules from outside the cell keep flowing inside it to dilute the cell’s solution.

This causes the cell to swell and burst. Finally, isotonicity is the case when the content concentrations inside and outside the cell are balanced, requiring no movement of water molecules so there’s no change in cell volume.

The answer to why saltwater fish cannot survive in freshwater lies in its tonicity. Seawater or saltwater has a much higher salt content that the fish living in it. In other words, it is hypertonic to the fish. Hypertonicity causes the fish bodies to lose water to the surrounding water through osmosis. Saltwater fish, therefore, drink a large amount of saltwater to compensate for this water loss saline waters. The excess amounts of salt are filtered out from their bloodstream through their kidneys and gills through urination.

If saltwater fish are put in a freshwater aquarium, it’s never the same. The problem arises from hypotonicity. Saltwater fish have higher salt content in their bodies that their surrounding freshwater.

This means that water from the surroundings will continuously flow into their bodies. Urination won’t help in this case because the cells of saltwater fish will saturate way faster than its kidneys can handle or its ability to urinate.

Their cells swell up and may burst, leaving them dead.

Can Freshwater Fish Survive in Saltwater?

Let’s also discuss how a freshwater fish would respond if it’s put in saltwater.

To dilute the highly saline surrounding water, water molecules from the bodies of freshwater fish will constantly move out to the surrounding waters.

Saltwater is too salty for freshwater fish to drink. With severe water loss, the freshwater fish die of dehydration.

Like its saltwater counterpart, freshwater fish also produce large volumes of urine, but it is low in salt. Since they live in low saline waters, their kidneys don’t need to work overtime to maintain the salt concentration in their bodies.

Fish That Can Survive in Both Saltwater and Freshwater

Some fish species, known as euryhaline fish, are versatile enough to survive in both freshwater and saltwater aquariums.

According to the National Biological Information Infrastructure (NBII), these fish are highly tolerant to salinity levels, making them extremely adaptable to both the settings.

This extraordinary quality allows them to migrate between saltwater and freshwater such as salt-rich seas and some freshwater rivers.

It is believed that fish adapted to different water settings due to the competitive advantage each offered. When it comes to euryhaline fish, it is thought to move from saltwater to freshwater or vice versa to escape from predators or inappropriate temperatures or to access more food.

The two major species of euryhaline fish include catadromous and anadromous. Catadromous fish spend most of the time in freshwater but migrate to saltwater habitats to spawn.

The National Marine Fisheries Service (NMFS) categorizes the European and North American eels under catadromous fish.

In contrast, anadromous fish spend the major part of their lives in saltwater but return to freshwater to spawn, where they were born.

According to the Alaska Department of Fish and Game, these fish include smelt, striped bass, sturgeon, salmon and shad.

While euryhaline fish can survive in both water settings, sudden changes in salinity levels of the surrounding water can still hard them.

When they migrate between saltwater and freshwater, they undergo an acclimation period that allows their bodies to adapt to the changing salinity in their surrounding waters. What helps them switch their salt balance physiology is the brackish estuarine environment.

This environment lies in the middle of freshwater and saltwater habitats.

NBBI states that euryhaline fish balance the salt concentrations between their bodies and the surrounding waters by associating their different stages of life with the salinity levels of various habitats.

Habitats of Saltwater Fish

If you happen to look for saltwater fish directly in their natural environment, make sure you stick to the following three types of marine environments because these are the habitats where they’re commonly found:

Pelagic Zone

The pelagic zone is used to refer to the migratory route or water column through which pelagic saltwater fish frequently swim.

They’re neither found near the shore nor the close to the bottom. Popular examples of pelagic fish include wahoo and tuna.

Coral Reef Systems

These are fish habitats rich in the coral reef.

They are common habitats for reef fish such as the highly sought gag grouper and red snapper.

Demersal Zone

These are part of seas or lakes along or close to the respective sea floors or lakebeds. Demersal fish are common saltwater fish that stay close to this zone.

Among the most common species include halibut and flounder.

Most Popular Saltwater Fish

Let’s now explore the most common saltwater fish:

King Salmon

Also known as Chinook, King Salmon is one of the most sought-after fish that’s found along the West US Coast.

Striped Bass

Along the East US coast exists the favorite migratory species called the Striped Bass. These are relatively sleeker than the White Bass.

Pacific Halibut

Halibut, the largest Pacific flathead, is another well-known saltwater species.


Found between the East coast of the Atlantic and the Gulf of Mexico, Redfish is often regarded as the high fighter among saltwater fish species.


Bluefish is another migratory found along the East Coast. It’s particularly known for its sharp teeth.


To sum it all up, saltwater fish can’t survive in freshwater because of the high salt concentration in their bodies.

The reason for this is osmosis that takes place according to the tonicity of the surrounding water. The same can be said about freshwater fish, which cannot adapt to a saltwater environment either.

The sensitivity of fish to the water they live in explains how the slightest fluctuations in water salinity can impact their very survival.

Novice aquarists need to fully understand the biology of fish before setting up a saltwater aquarium.

You can always resort to owning euryhaline fish if you don’t want to put in all the effort to maintain the right water type for either of the saltwater or freshwater fish.

However, make sure the aquarium is not subject to abrupt salinity changes that can pose threats to euryhaline fish too.

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