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Pool Doctor - June/July 2022

Black stains on the pool walls... We can help


Black stains on the pool walls... We can help



We frequently get asked by pool owners to refurbish their pools. Upon arriving at the pool, we find green and black stains on the pool walls and in the corners – algae, an aquatic organism which seems to reproduce during the rainy season.

In most cases, the high alkalinity in the water (high PH levels) causes a build-up of “white stain marks” on the pool walls. During summer, this process is normally accelerated by the fact that the water is warmer (almost like scale build-up inside a kettle). In the event of the chlorine level being too low for a period of time, it makes an ideal environment for the little green algae particles to start growing. They are initially difficult to see as they “hide” in the porous surface of the lime. By the time you do see them, a very big effort is needed to get rid of them. These algae may appear black in the pool, but when you retrieve a sample and rub it on a piece of white paper, you’ll notice a greenish tinge. Black alga is a super resistant and hard-to-kill algae strain. These spots are pin-head to coin-sized. Black algae have hard outer shells that protect the cells inside. Pools with porous surfaces like concrete, gunite and exposed aggregate are more susceptible to black algae because they can penetrate and grab hold of the walls. Removal can be difficult, but not impossible with the right tools.

Initial DIY remedy:

  • Lower the PH level in the pool to 7.2 and alkalinity to 80 -120 ppm.
  • Increase the chlorine level in the pool to more than 3.5.
  • Do daily brushing of the areas where you can see the black algae.
  • Please visit the Lic Pool shop to get the correct brush. A steel or metal wire brush will cause lots of damage to the pool surface and cannot be used.
  • Keep the pool cover open.
  • Swimming during the treatment of black algae is not advisable due to the high chemical content of the water.
  • Do not dump large volumes of acid into the pool. If the pool finish inside the pool is already old and worn, these strong chemicals may cause a lot more damage to the pool in general, than just a few black algae spots.

If you do not get the needed results, I would advise you to email us at [email protected] We do these repairs very regularly and will be able to help you get rid of the problem, without destroying the pool.

Until next time.

Burgert Terblanche

Tel: 061-232839, Fax: 061-238602
Email: [email protected]


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