system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't saltwater fish survive in fresh water?
gpt
Simply put, saltwater and freshwater fish have different systems for maintaining the right amount of water in their bodies. Think about salt, if you sprinkle it on a slug it makes the slug dry out, killing it. The same thing would happen to a freshwater fish if you plonked it in the middle of the sea. saltwater fish deal with this by clinging onto urea in their bodies. Urea is normally something we want to excrete because it's a waste product that gets produced by the detoxification of ammonia. Too much of even urea in your blood is very bad for you, let alone the deadly ammonia. However, saltwater fish are specially adapted to deal with this. 

Let me explain something called 'water potential'. When you have something dissolved in water (a solute dissolved in a solution), we say that the water potential has been 'lowered'. The more that is dissolved, the lower the water potential. Water tends to move from an area of low to high water potential until it is balanced in both areas. The water actually moves both ways (its called a dynamic equilibrium) but eventually ends up with an equal concentration in both areas because of the random movements of the water molecules (the concentration of the solution = number of solute molecules per volume of water so larger volume of water needed for more solute).

By having urea in their cells, the water potential of the cells is roughly the same as the water potential outside the cells (caused by salt in the salt water) so no water movement occurs. hence, water does not gush out of the cells and leave the fish all dried up and dead. 

Freshwater is 'purer' so has a higher water potential meaning that water would move into the fish cells, making them burst. Some fish can survive both environments due to different biochemistries but properly saltwater animals like sharks (excluding the bull shark) have this urea retention mechanism.