According to the Office of Science at the Department of Energy, "Approximately 2 percent of all 21,000 species of fish actually move from fresh water to salt water or from salt to fresh at some point in their lives"(www.newton.dep.anl.gov). notable examples are eels and salmon. To accomplish this they have special triggers in their body that help them with the excretion and absorption of salt. Most fish, however, would die from losing or gaining water in their bodies at a rapid rate. To better understand this we need to understand the process of diffusion and osmosis.
Freshwater Fish in Salt Water
Diffusion as defined by Encarta is "the flow of energy or matter from a higher concentration to a lower concentration." So if our freshwater fish were placed in salt water, the water molecules contained in the fish's body would pass through the cell membranes of the fish, causing the fish to lose water. This sudden change in the molecular makeup of the fish would cause the fish to die.
Osmosis as defined by purchon, www.purchon.com, is "the passage of water from a region of high water concentration to a region of low water concentration." If our saltwater fish were placed into fresh water, the opposite of diffusion would happen. The salt molecules in the fish would begin to absorb the water molecules from the fresh water, causing the fish to gain large amounts of water weight. This sudden change in the molecular makeup of the fish would kill it as well.
While there are some fish that will be able to move from one type of water to the other, most fish can not. Freshwater fish and saltwater fish have evolved over time to survive in their specific water conditions. Trying to maintain a certain type of fish in a different water parameter will likely result in the death of the fish.