The main difference between freshwater fish and saltwater fish is essentially the total diversity of the media in which they live. When two liquids with different salt concentrations are separated by a membrane, a problem arises. In our case, a liquid is water in which the fish live, the other is the blood of the fish and the membrane which separates them is the skin of the fish. In freshwater fish are more concentrated than the liquid that circulates in the fish, to avoid bursting due to water absorbed by the skin, must emit large amounts of liquid every day in the form of very concentrated urine. This state of things is reversed in marine fish, swimming in a liquid concentrate much of their blood, so they have a continuous passage of fluid from their blood in seawater. The marine fish must then drink copiously to replenish the fluid lost, and the salts that accumulate are excreted in urine, but very little focused. Some marine fish have as a product of excretion of urea and not ammonia, which is stored in their body to increase the salt concentration of their liquid until almost to approach that of the surrounding water. This acts as a stabilizing salt, as a mechanism-buffer, so that some fish of this group can move from salt water in brackish and even fresh water without much difficulty.
The fish inhabit the waters on our planet, water that is usually divided into sweet, salty and brackish. The difference lies in the quantities of dissolved salts per liter. Over time the fish have adapted to different environments, responding to biological and physico-chemical rules of nature.

However there are fish that can change the type of water during life, from sweet to savory and back to breeding grounds or food, like salmon, eel and others who live in the lagoon waters, can afford to go in the sea or rivers that flow into the lagoon (some puffers etc).