What happens to a freshwater fish when it's put in saltwater?

We often use freshwater as a treatment for saltwater fish and vice versa. Some fish live their lives, at different stages, in one then the other. Some fish move back and forth on a more regular basis and some fish are either completely freshwater or completely saltwater. So, when you put that saltwater fish into freshwater here is how it goes. read more

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. read more