When a fish is moved from salt water to fresh water, the cells of the fish will swell due to overhydration. Cells of fish are semi-permeable (allows only some molecules to pass through them). Large amounts of fresh water around the fish will move into the higher salt concentrations of the fish's bodily fluids through osmosis, and there will be over functioning of the fish’s kidney to retain excess water the fish does not need. Also, essential electrolytes within the fish are pushed outside, and this will result in the death of the fish.
