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.Continue Reading
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.Learn more about Fish