Split by moderator from HERE as it perpetuates a myth and contradicts with our teachings. jblizzle
Shut off your pump and let the water go still. In the deepest area of the pool add muriatic acid in a steady pour to create a column of acid in the pool. This will eat up alkalinity and lower the ph. If you have a plaster pool do this over the main drains, if it is shallow pour several columns around perimeter an arms length from the wall. Acid is heavier than water and too much too fast could hit the bottom of the pool depending on depth. Wait about 5 minutes and turn the pumps back on. Wait for the water to stir a bit then test again. Always seems to work for me.