Why Does My Pool Have Red Spots?

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Swimming pools can develop a number of stain problems.
Swimming pools can develop a number of stain problems. (Image: Stockbyte/Stockbyte/Getty Images)

Owning a swimming pool offers many opportunities for fun with family and friends but also adds to the list of routine home maintenance tasks. Regular monitoring of pool chemistry, the addition of the proper pool chemicals and scrubbing pool surfaces keep water sanitary and inviting. But even with regular care, occasional algae growth or iron deposits can cause red spots on pool walls and floor.

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About Red Algae

Red spots in the swimming pool can be caused by red algae, which is not actually an algae but a type of bacteria that can begin to grow on pool surfaces and can expand to an unsightly problem. Red algae can also appear as rusty-brown spots or bright pink patches on the walls. This problem requires vigorous treatment of the pool water, including adjustment of pool chemicals and other actions to remove the growth from the water.

Treating Red Algae

Wear latex gloves and goggles when working with pool chemicals. Adjust the pool chemistry to the recommended range for your pool. This adjustment should include total alkalinity, pH and water hardness. The pH range should be between 7.0 and 7.2, according to the CleanPool&Spa website. Adjust alkalinity to 80 to 100 parts per million and hardness to 150 to 250 parts per million. Shock the pool with granular calcium hypochlorite with an amount 10 times what you ordinarily add to the pool. Brush the walls, floor and stairs of the pool vigorously with a stainless steel brush and run the filter for 24 hours to remove the red algae from the water. Backwash the filter twice a day until the red algae is no longer evident. Rinse the filter carefully to remove the bacteria from the surfaces. Add a commercial algaecide specifically formulated to remove red algae and continue filtering until no sign of the algae remains.

About Iron Deposits

In many areas of the United States, water contains high concentrations of iron. These iron particles can precipitate out of solution and cling to the walls and floor of swimming pools, creating small red or brown spots that grow larger over time. The spotting creates an ugly appearance and can be difficult to remove if left on pool surfaces.

Treating Iron Deposits

To treat red spots caused by iron in pool water, first adjust the pH and alkalinity of the water. Add muriatic acid to lower the alkalinity and sodium carbonate to raise the alkalinity of the water. Give the stained area a good scrubbing. Add a sequestering agent available at pool supply stores. This chemical will bind with the iron particles so they can be easily filtered out of the water. Run the filter for 12 hours and re-test the alkalinity and pH to see whether it needs re-adjustment, according to the Lamotte Pool Service site.

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