Mustard Algae


- if you detect mustard algae - act swiftly to prevent an expensive disaster from occurring

Mustard algae

This type of algae is commonest on the shaded parts of the pool, typically on north-facing walls, under diving boards, etc.

Once established Mustard Algae is difficult to shift and almost impossible to completely eradicate it with normal levels of Chlorine

This is not because it is a particularly hardy type of algae,  nor that it is hard to kill - it's just hard to kill all of it because it can survive in the tiniest crack or imperfection of the pool surface and it often reappears quite soon after it has been treated


consists of thoroughly cleaning the pool and application of appropriate chemicals


  • This ensures that there is a high Chlorine level in the pool so that when you carry out the next step any algae dislodged from the surfaces into the water will be killed
  • Remove every visible spot of algae by scrubbing -  using a good quality pool brush with stainless steel bristles
  • Do not use a stainless steel brush on a glass-fiber or liner pool as it may damage the gel-coat or liner
  • If the water has become cloudy after the scrubbing use a flocculant and vacuum the sediment to WASTE the following day
  • Make up a 4:1 bleaching solution, in a plastic bucket or garbage can, with 4 parts water to 1 part liquid Chlorine (Sodium Hypochlorite)
  • Use this mixture to bleach everything possible 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, 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 any filter with the mixture as above
  • Apply a Phosphate Starver solution to strip phosphates (algae food) from the water
  • Apply half a litre of Hydroghen Peroxide 110-vols per 50m³ of pool water (available from a pharmacy) via the skimmer with the pump on, If you can't find it at this concentration, apply 5 litres of 30-vols per 50m³.
  • Apply Copper Sulphate, at a rate of 3 grams per m³ (4 ounces per 10,000 US Gals), pre-dissolved via the skimmer with the pump on
  • Vacuum the pool, check and adjust pH, and superchlorinate.
  • Ensure that Chlorine is kept at a proper level to prevent another attack
  • Breathe a huge sigh of relief - and remember that the price of a clean, algae-free pool is eternal vigilance!

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Ken Walker - MyPoolGuru©