When you notice a stain in your pool you should attack the problem right away. The longer you wait to get a stain out the harder and more costly it will be. Gunite pool stains come in an array of colors and are caused by algae, heavy metals, organic matter and more. You must determine the source of the stain in order to properly remove it. Determination can be made by examining the color of the stain.
Brown Organic Matter Stain
Lower the water level to just below the stain. Organic matter stains are almost always found on the pool floor so you will have to completely empty the pool to get to the discoloration.
Place some dry chlorine in an old sock and press it against the stain.
Rub the sock against the stain for a couple of minutes to remove the greenish brown color.
Black Algae Stain
Close off the pool to swimmers and animals before you begin cleansing. The pool will need to be closed off for two weeks to prevent diarrhea and illness due to the high amount of chlorine which you will add to your pool in step 3.
Decrease or increase the pH level of the pool to around 7.0. You can obtain a pH test kit from your local pool supply store. Follow the directions that come with the test kit to determine whether you need to raise or lower your pool's pH. You can raise the pH level with pH Up and lower the level with pH Down. Both of these products can also be found at your local pool supply store.
Add one gallon of liquid chlorine to the pool at a time until the chlorine level reaches 10 parts per million or PPM. A 20,000 gallon pool will require between 5 and 8 gallons of liquid chlorine. Make sure that your pump is running while you add the chlorine. After each gallon check the chlorine level with a chlorine test kit found at your local pool supply store.
Run the pool pump for at least 24 hours for best results.
Scrub the sides of the pool to loosen up the black algae and allow the pool filter to work the algae out. You will need to back wash a few times during the 24-hour period. Remember to replenish the diatomaceous earth (DE) if you have a DE filter.
Continue to run the pool filter as normal once you have completed the chlorine bath. The pool should be safe to open back up after the chlorine level has been reduced to normal and two weeks have passed.
Dark Heavy Metal Stain
Lower the water level to just below the stain.
Scrub the stain with a hard-bristle brush and natural ascorbic acid. The ascorbic acid should remove almost any heavy metal stain instantly. Heavy metal stains come in many colors including red, brown, green, and blue, purple, and pink to almost black.
Prevent heavy metal stains from returning by using 50 percent softened water to fill your pool.