Why are fishes not able to survive in distilled water?

  Take a fresh grape fruit and keep it in distilled water for an hour. it is stouter than before. Another fresh grape fruit kept in salty water for an hour will appear slim and wrinkled. The grape has dissolved minerals,sugars and other ingredients in its juice. The skin of the grape is a semi permeable membrane across which water can diffuse from one side to the other through osmosis. When the grape is kept inside distilled water, the concentration(salinity) of juice in the grape is higher than that of water and water moves osmotically from outside the grape to inside. Hence the grape appeared stouter. The reverse happened in the case of salty water.

  The skin of the fish is like the skin of the grape fruit. If the fish is in distilled water, there is an osmotic flow of water from outside the fish to the inside because the salinity of the fish body is higher than that of water. Thus there is danger of dilution of the body fluids of the fish or at worst that of being blown out if the fish is to be in distilled water for a very long period of time. Courtesy : The Hindu