Why would a saltwater fish die in freshwater?

This is all due to osmosis. If a Saltwater Fish is placed in freshwater, its polar molecules, in this case salt, attracts the other water molecules around the fish, which has no polar molecules, for it is fresh water. The water molecules enter the body so they can hang out with the salt molecules, and enter the fishes cells through the cell membrane, which is semipermeable, which means that only certain molecules, such as water, can pass through it. When the water enters the cells, which are already filled with water, they become hypertonic, and burst, causing the fish to die.