There are no clear identifying features of saltwater fish compared to freshwater fish. The main difference between saltwater fish and freshwater fish is the regulation of salt and water in their bodies, which happens on a cellular level.
Since they live in a salty environment, saltwater fish must be able to pump excess salt out of their bodies.
Their kidneys filter sodium and produce concentrated urine. In addition, their gills contain chloride cells that push the salt out of the fish's tissues and into the saltwater. This differs from freshwater fish that usually live in low-salt environments. Freshwater fish have gill cells that pump nutrients, such as sodium, chloride and calcium, into the fish.
Saltwater fish enjoy a larger and more stable environment than freshwater fish; therefore, they are more sensitive to changes in the size of their environment and water conditions, such as temperature and pH. Because of their large habitats, saltwater fish have the capacity to grow much larger than freshwater fish. For example the largest living saltwater fish, the whale shark, grows up to 40 feet long and can weigh more than 20 tons. The largest freshwater fish is the beluga sturgeon thta reaches 15 feet long and weighs more than 2,500 pounds.