What Is the Difference Between Saltwater & Freshwater Fish?

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Although a few species of fish can live in either salt or freshwater, the great majority require a specific environment. The composition of water determines where saltwater and freshwater fish can live. Understanding the difference between saltwater and freshwater fish is important if you plan to keep fish at home, as each requires the correct environment and the right care to survive.


Saltwater is the environment all fish survived in until geologic events, such as earthquake and volcanic activity, created conditions that isolated groups of fish. New habitats, including freshwater, and food sources required these fish to adapt or die. Evolutionary adaptations, including time and natural selection helped create new freshwater fish species.


An aspect of fish physiology called osmoregulation highlights a major difference saltwater and freshwater fish. Osmoregulation refers to how to fish control water flow across their bodies and includes the composition of body tissues, gills and kidney function.

Body tissues in a saltwater fish contain less salt than the water in which it lives. Because the saltier environment in the outside water draws water from body tissues, a saltwater fish constantly loses water through its skin and gills. To compensate and prevent dehydration, a saltwater fish drinks large quantities of saltwater, produces very little urine and secretes salt from this water through its gills.

In contrast, body tissues in a freshwater fish contain more salt than the water in which it lives. As a result, water continually flows into the body of a freshwater fish through its skin and gills and the fish has no reason to take in additional water by drinking. Freshwater fish avoid an excess of water in body tissues by producing large amounts of urine.


Freshwater fish can be cold-water fish or tropical fish. Cold-water fish are mainly goldfish, while tropical fish that can live in warmer water include many varieties. Examples of tropical freshwater fish include angelfish, discus, cichilds, barbs and cory cats. Saltwater fish are all cold-water fish. Examples include clown fish, butterfly fish, eels, seahorses, and lionfish.


According to AquariumSource.com, the lakes, rivers and streams that form the natural environment of a freshwater fish undergo constant change. This creates a fish more adaptable to changes in both a natural and aquarium environment, which is not the case with saltwater fish. Saltwater fish require an environment with little fluctuation in temperature, salinity, ammonia, nitrate and pH levels. The pH of a saltwater aquarium must stay between 8.2 and 8.4, according to Aquacon.com, while pH levels in a freshwater aquarium can range from 6.0 to 9.0.


Freshwater aquariums are less expensive than saltwater aquariums. The average cost to set up a freshwater aquarium, not including the fish, is approximately $270.00, according to FishLore.com. In contrast, the average cost to set up the same size saltwater aquarium is approximately $635.00.