There is a difference in their anatomy and physiology, and that is the way fish regulate water and salts in their internal cells. Most freshwater and saltwater fish maintain a level of salt in their blood, but these levels are different for the two types. The chloride cells of freshwater fish are designed to pump sodium, calcium and chloride into the fish. Freshwater fish are generally more adaptable to their environment and more "stable" than saltwater fish. (Freshwater fish). Body tissues are a huge difference between these types of fish (osmoregulation), and this shows a major difference between freshwater and saltwater fish.