If a saltwater fish is placed in freshwater, what happens to its cells?

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When a saltwater fish is placed in freshwater, the surrounding water has a lower solute concentration than the fish’s body fluids. Osmosis drives water across a semipermeable membrane from where there is more water (lower solute) to where there is less water (higher solute). Because the external environment is hypotonic relative to the fish’s cells, water moves into the cells, causing them to swell. This influx can stress tissues, and the fish’ body systems will work to compensate by excreting the excess water and adjusting salt balance. In short, water moves into the cells by osmosis, so the cells swell.