When a freshwater fish is placed in salt water, the most likely event that would occur is: water would be pulled out of the fish's cells, causing dehydration.
This happens because the concentration of salt outside the fish is higher than inside its cells, leading to a process called osmosis. Water moves from an area of lower solute concentration (inside the fish) to an area of higher solute concentration (the salt water) in an attempt to equalize the solute concentrations on both sides of the membrane. As a result, the fish loses water, which can lead to dehydration and potentially harmful effects.