The difference between saltwater and freshwater fish is that saltwater fish need to prevent hyponatremia and freshwater fish need to prevent dehydration. This is because water tends to move from a place that has a lesser concentration of salt to one that has a greater concentration of salt.
In saltwater fish, there is more salt in the water than in their bodies. To compensate, the fish need to excrete much of the salt they ingest. Some have gills that can remove the salt from the water and excrete it.
Freshwater fish get salt from their food and excrete great amounts of urine. Their kidneys are also good at retaining salt.
According to Scientific American, salty environments draw a large amount of moisture from a fish's body tissues, causing it to constantly lose water through its gills and skin. In order to stay hydrated and survive, saltwater fish must consume very large quantities of saltwater. This causes them to produce very little urine, and they also secrete large amounts of salt through their gills.
A fish that lives in a freshwater environment has a normal balance of salt in its body since its environment does not constantly draw salt from its tissues. As a result, freshwater fish do not become dehydrated and produce large amounts of urine. Freshwater fish do not need to consume additional water through drinking.
There are very few species of fish that are able to live in both saltwater and freshwater environments. According to LiveScience, most fish are highly sensitive to salt-level chances that occur where they dwell.