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The solvent will flow from high(hypertonic) to low(hypotonic) concentration. The fish that is used to salt water has high solutes (Salts) and the freshwater is devoid of solutes. As a result, when you place a saltwater fish in freshwater, the freshwater will move into the cells. This will cause them to swell and burst (become turgid)!!! Not a nice thing to do to a fish!
Why do saltwater fish die when placed in fresh water?