Answer
A saltwater fish placed in a freshwater tank will have its cells expand to the point of lyse.
A freshwater fish placed in a saltwater tank will have its cells shrink to the point where the cells can't function.
Both cases you care taking cells and placing them in non-isotonic solutions.
Work Step by Step
The saltwater fish is used to living in a higher molarity solution. Freshwater will be hypotonic to the fish. When placed in freshwater, the saltwater fish's cells will take in water and expand.
The freshwater fish is used to living in a lower molarity solution. Saltwater will be hypertonic to the fish. When placed in saltwater, the freshwater fish's cells will release water and shrivel up.
