Dear Rohini, the pH level for a ideal water should be between 6.5 and 7.5. A changing pH in water can be an indicator of increasing pollution or some other environmental factors. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. It also affect the organisms living in the water.
International agencies recommend that drinking water have a pH between 6.5 to 8.5. Hard water, or water with a high pH, contains a high concentration of dissolved minerals. Although it is healthy for the body (The World Health Organization says that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans), minerals such as calcium can cause problems with its positioning on ceramic surfaces and its sour taste. Mineral deposits in bathtubs and household devices can leave residues in flakes. Soft water, or with low pH, tends to carry large concentrations of metals such as manganese and iron. These metals can be corrosive potentials of pipes in old houses.
On the other hand, maintaining a balanced pH in the water is critical for healthy aquatic life. Fish and other organisms depend on the high quality of the water with the right amount of dissolved oxygen and its nutrients. A high or low pH can break the balance of water chemicals and mobilize pollutants, causing toxic conditions. Aquatic organisms may experience problems causing populations to decline. For that reason, water quality scientists generally analyze the pH as a control parameter for water quality.