Differences between Freshwater Fish and Saltwater Fish


There are many different species of fish, and they can be broadly categorized into freshwater fish and saltwater fish. Many freshwater fish can be found in rivers and lakes. Saltwater fish are found in the ocean. Together, there are over 30,000 different species of fish, and many new species are being discovered each year. This makes them the most diverse group of vertebrates.

It is believed that life began several billion years ago in the ocean. Therefore, early fish were all saltwater fish. Over time, some of these ocean-dwelling creatures were able to take advantage of untapped resources, like freshwater rivers and lakes. Moving into bodies of freshwater meant that they would have to adapt to their new environments. Some of these adaptation became permanent, and led to species differentiation.

Difference Between Freshwater and Saltwater Fish
Freshwater and Saltwater Mix in Estuaries

While marine waters cover more than 70 percent of the earth’s surface, freshwater covers only 1 percent. Surprisingly however, 40 percent of the 30,000 species of fish dwell in fresh waters.

This is a very high proportion of species density given the small area that the bodies of freshwater actually covers. This data indicates that there are approximately 1 species of fish for every 15 cubic kilometers of freshwater and 1 species of fish for every 100,000 cubic kilometers of marine waters. Constantly changing environments and ease of geographical separation in small bodies of freshwater habitat have resulted in a high degree of diversification of freshwater fish. The constantly changing environments have also forced freshwater fish to become more adaptive to their environment. In comparison, saltwater fish have been able to thrive in a relatively more stable environment in a larger ocean environment. Therefore, freshwater fish are generally more adaptable to a wider range of water parameters compared to saltwater fish.

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Freshwater Fish vs Saltwater Fish

Surprisingly, there are no significant anatomical differences that universally distinguishes freshwater and saltwater fish. However, there is a key physiological different between freshwater and saltwater fish. The difference between saltwater and freshwater fish can be found in how they regulate water and salts in their internal cells, also known as osmoregulation.

Osmoregulation in Fish

Freshwater fish and saltwater fish regulate water and salts in their internal cells differently. Saltwater fish loses salt through their skin, while freshwater fish tend to absorb it.

Osmoregulation in Freswater Fish. NOAA. Image modified by Biezl.

Water is always trying to reach equilibrium. Therefore, the salt moves from high concentration areas to low concentration areas. Since ocean water is very salty, saltwater fish will have a lower concentration of salt in their body compared to the water they swim in. As a result, most saltwater fish constantly lose water through their gills and skin. In contract, freshwater fish will constantly absorb water through their gills and skin.

Osmoregulation in Saltwater Fish. Image modified by Biezl / [CC BY-SA]

Most freshwater fish and saltwater fish maintain a salt concentration in their blood of approximately 10 parts per thousand (ppt), or 10 grams of dissolved salt per liter of water. Since freshwater fish swim in water with approximately 0.5 ppt, the chloride cells in their gills are designed to pump sodium, calcium and chloride into the fish. On the other hand, since saltwater fish swim in water with approximately 35 ppt, the chloride cells in their gills are designed to pump salt out of the out of the fish. This process of controlling the flow of water across their body is called osmoregulation.

What Happens if you put a Saltwater Fish in Freshwater?

Saltwater fish have hypotonic cells, which keeps the salt inside the cells. Therefore, when saltwater fish are placed in a salt deficient freshwater environment, water will rush into their cells. This will cause the fish to die.

What Happens if you put a Freshwater Fish in Saltwater?

Freshwater fish have a higher salt concentration in their body than the water around them. Therefore, they are constantly working to keep the salt in their body, and expel the water out of their body.

If the freshwater fish is placed in a saltwater environment, the salt would flood into their body at a high concentration. This would dehydrate the fish, and kill the fish.

Which Fish Can Live in Freshwater and Saltwater?

Some fish are able to live in both freshwater and saltwater. Some examples of these fish are:

  • Sockeye Salmon
  • Bull Shark
  • Barramundi
  • American Eel
  • Atlantic Stingray
  • Green Sawfish
  • Gulf Sturgeon

Euryhaline Fish

Fish that are able to live in both freshwater and saltwater are called euryhaline fish. These highly adaptable fish are able to migrate back and forth between the ocean and rivers.

There are two different types of euryhaline fish.

Anadromous Fish

Euryhaline fish that spawn in freshwater and migrate to the ocean are called anadromous fish. An example of anadromous fish are salmon.

Catadromous Fish

Euryhaline fish that spawn in the ocean and migrate into bodies of freshwater are called catadromous fish. Examples of catadromous fish are North American eels and European eels.

While euryhaline fish are adaptive species, they do require a acclimation period to allow their bodies to adjust to the different salinity levels. These fish may gradually acclimate their body in estuaries, which is a region where freshwater and saltwater mix.

Stenohaline Fish

Fish that are only able to live in a narrow range of salinity are known as stenohaline species. The majority of fish are stenohaline species. Unlike euryhaline species, stenohaline fish are unable to adapt to different salinity levels. For example, goldfish can only live in freshwater and clownfish can only live in saltwater. Both fish are stenohaline species.

Freshwater Aquarium vs Saltwater Aquarium

When choosing between a freshwater aquarium setup and a saltwater aquarium setup, it is important to realize the difference in level of difficulty and level of care required.

Difficulty of Care

In general, freshwater fish are hardier due to their adaptive nature. Even as a stenohaline species, most freshwater fish will accept of a wider range of water parameters, compared to saltwater fish. In the ocean, the salinity level and water parameter does not change as often as rivers and ponds. Therefore, they require more specific and stable water conditions.

Cost of Fish and Equipment

While market value of fish can vary depending on the season and distance from the coastal lines, marine fish are much more expensive compared to most freshwater species. Due to the additional equipment involved in a saltwater aquarium and the price of the fish itself, a saltwater aquarium setup can often be twice as expensive as a freshwater aquarium setup.

Freshwater Aquarium Setups

Since freshwater fish of often cheaper and easier to take care of, most beginner fishkeeper start the hobby with a freshwater aquarium. Just like saltwater fish, there are many colorful and attractive freshwater fish including cardinal tetras, fancy guppies, killifish, bettas, and cichlids, just to name a few. While they are not recommended for beginners, discus fish is another very attractive freshwater fish with a variety of coloration.

Even within the scope of freshwater aquariums, there is a great variety of tank setups available. Freshwater tropical community tanks are one of the most popular aquarium setups for both novice and advanced fishkeepers. While it is relatively inexpensive to set up and maintain, one can also appreciate a great variety of fish in a single aquarium. However, it is important to understand the requirements of each species in the community in order to ensure compatibility. Since different species have different swimming patterns, combining top level swimmers, middle level swimmers, and bottom dwellers can be an attractive effect in a community tank.

Basic components in a freshwater aquarium setup includes filtration, lighting, and heating. For freshwater aquariums without a heating mechanism, coldwater fish are suitable. Coldwater fish includes goldfish and white cloud mountain minnows, among many other small river species.

As a general rule of thumb, it is advisable to provide at least 1 gallon of water for 1 inch of freshwater fish.

Saltwater Aquarium Setups

There is a great abundance of colorful marine fish with striking patterns and body formations. While different species have varying requirements, saltwater aquarium setups can be categorized into three basic types.

The most basic marine setup is the fish-only (FO) saltwater aquarium setup. This type of aquarium setup is well suited for novice saltwater fishkeepers due to its simplicity. FO aquariums are decorated with coral replicas instead of live rocks. Hardy marine fish such as Damselfish, as well as other tank-bred species, are great candidates for FO aquariums.

Another type of marine tank setup is fish-only-with-live-rock (FOWLR) saltwater aquariums. Live rocks, which are pieces of mature coral reefs, allows colonization of a various marine life such as invertebrates and sponges. Since live rocks house an abundance of beneficial bacteria, it aids in filtration and maintaining desirable water parameters. Since live rock require specific acclimation techniques, lighting specifications, and supplements, FOWLR saltwater aquariums require a higher degree of care than FO saltwater aquariums.

The most challenging type of aquarium, more so than FOWLR aquariums, are reef aquariums. In reef aquariums, the primary focus is placed on the corals, invertebrates, and anemones. While fish can be present in a reef aquarium, they are considered as an accessory of the entire setup. Reef aquariums require very specific water parameters, lighting conditions, and water flow that must be maintained on a very regular basis. Thus, reef aquariums are generally reserved for the most advanced aquarists.

Finally, as a general rule of thumb it is advisable to provide at least 5 -10 gallons of water for 1 inch of saltwater fish.

Brackish Aquarium Setups

Brackish water aquariums lies between freshwater and saltwater aquariums. Difficulty in maintaining a brackish aquarium depends on the type of fish that is being housed. For novice fishkeepers, hardy fish such as Mollies are recommended for brackish aquariums. Just like a saltwater aquarium, a hydrometer is necessary in order to monitor the salinity in brackish aquariums. Whenever there is a change in salinity, it should occur gradually in order to allow the fish to properly acclimate.

Fish Laboratory

With decades of collective fishkeeping experience, we are happy to share the knowledge that we've picked up along the way. Our goal at Fish Laboratory Aquatics is to keep publishing accurate content to help fishkeepers keep their fish healthy.

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