Black Algae

black algae

Black Algae is very bad news for a pool-owner or a pool-user.  Not only is is ugly and VERY difficult to eradicate, it can also cause respiratory problems when spores are inhaled in tiny droplets of pool-water.

WARNING Black algae spores can grow in the the lungs, nice and safe in a warm, chemical-free environment!

Black algae spreads in several ways.  

  1. It grows laterally along the grout lines of a tiled pool or on an cement surface, pool-liner or painted pool.
  2. It spreads by releasing water-borne spores, which colonise other areas of the pool.
  3. Water that is splashed out of the pool, will evaporate and the spores can then be trodden or blown to different areas.
  4. It can (and IS) transported from pool-to-pool on swimming costumes and other items like arm-bands and goggles,etc.


If you detect black algae - also known as blue-black algae - act swiftly to prevent an expensive disaster from occurring

Black algae form a layered structure where the first layer (which Superchlorination may kill) protects under-layers from further destruction

Therefore a pool afflicted with this type of algae needs a lot of work to eradicate it completely


Treatment for a moderate attack of black algae 

If there are only one or two patches of black algae it is sometimes possible to eradicate it without draining the pool: -

  • Apply Copper Sulphate, at a rate of 3 grams per cubic metre, via the skimmer
  • Do not overdose with Copper Sulphate
  • Check pH and adjust as required
  • Superchlorinate to 10 ppm
  • Brush the algae spots with a stainless-steel pool brush to remove all visible signs
  • Turn the pump off and use crushed Chlorine tablets, contained in a cloth bag or an old sock, and tied to a length of cord, to spot treat the algae (not recommended for vinyl-liner or painted pools)
  • On cement or tiled pools black algae spots can also be rubbed with a Chlorine tablet -  wear gloves for doing this
  • Pour a good concentrated all purpose algaecide on the algae spots and leave it in contact with the algae overnight
  • Turn the pump and filter back on, then brush dead algae from the surfaces
  • Brush the black algae each day until it is no longer visible
  • In severe cases repeat treatments will often be necessary
  • Ensure that Chlorine levels are maintained to prevent further attacks


Treatment for a bad attack of black algae 

  • The best solution to eradicating black algae is to drain the pool, acid-wash and jet-wash the tiles, then irrigate the entire surface of the pool with liquid chlorine (Sodium Hypochlorite)
  • Allow this to remain in contact with the tiles for at least an hour and then jet-wash the pool again
  • NOTE! - do not allow hydrochloric acid and sodium hypochlorite to mix or they will produce deadly Chlorine Gas.
  • Repeat acid- and jet-washing until every visible trace of Black Algae has been removed.


After the visible black algae has been removed from the pool: - 

  • Make up a 4:1 bleaching solution, in a plastic or rubber bucket, with 4 parts water to 1 part liquid Chlorine (Sodium Hypochlorite)
  • Use this mixture to bleach everything that is associated with the pool
  • This includes pool tools, auto cleaners and their hoses or cables, skimmer lids and baskets, pump strainer lid and basket, inside the pump body, the pool walls above the waterline, the pool-surround and patio, under the lip of a pool coping, floating toys, noodles and anything else you can reach
  • Thoroughly clean the filter cartridge or DE filter grids and then soak them in the same mixture as above
  • Don't forget to thoroughly clean the inside of the filter with the mixture as above
  • When the pool is re-filled, apply Copper Sulphate, at a rate of 3 grams per cubic metre, via the skimmer
  • Do not overdose with Copper Sulphate
  • Apply a Phosphate Starver solution to strip phosphates (algae food) from the water


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Ken Walker - MyPoolGuru©