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This discussion covers different types of algae and methods of treating them.


Algae requires sunlight, water and nutrients to grow.  The rate of algae growth is ultimately limited by sunlight and temperature regardless of nutrient level and in ideal conditions takes 3-8 hours to double in population.  Algae can be killed by chlorine alone, but it is the active form of chlorine, hypochlorous acid, that kills algae and this form is much lower in quantity in the presence of Cyanuric Acid (CYA).  In fact, the minimum chlorine level for residential pools is that required to kill algae and not that of killing pathogens that are mostly much easier to kill.


There are many different species of algae, but they can roughly be grouped into the following types with associated methods of treatment (some are not really algae and are noted as such).


  • Green Algae.  This is the most common type of algae and the easiest to kill.  Most species of this algae initially make water dull or cloudy before becoming visibly green but some species show up as localized green clumps while others give the water a cloudy green haze.  Algae can be distinguished from metals in that clear green water is almost always metals.  For manually dosed pools, a Free Chlorine (FC) level that is 7.5% of the Cyanuric Acid (CYA) level will prevent green algae growth in almost all cases.  For saltwater chlorine generator (SWCG) pools, an FC that is 5% of the CYA level will prevent green algae growth.  If one needs to shock the pool to kill an existing algae outbreak, then an FC that is 40% of the CYA level will kill the algae relatively quickly.  These levels assume good circulation.
  • Black Algae.  This algae is more chlorine resistant than green algae mostly because it creates a waxy surface layer that protects the algae that grows deeper into pool surfaces, most notably plaster.  This algae grows relatively slowly and its presence indicates extended periods of time with insufficient chlorine levels.  This algae can be prevented by maintaining the same FC/CYA ratios as needed to prevent green algae growth.  For an existing outbreak, one should shock with chlorine (at the same levels as shocking for green algae) along with using a brush to scrape off the waxy surface layer to expose the algae to high chlorine levels.  Black algae can be definitively identified by scraping some off and spreading it on a white piece of thick paper which should show it as dark to medium green.
  • Yellow/Mustard Algae.  This algae is about twice as chlorine resistant as green algae.  It does not like direct sunlight so is usually growing on the shady side of the pool.  It looks like dirt or powder but when brushed it produces a yellowish cloud.  It is often confused with pollen, but pollen will settle based on circulation patterns including areas of direct sunlight.  Yellow/mustard algae can be prevented by maintaining an FC that is 15% of the CYA level, but this is often impractical so the usual approach is to completely get rid of the algae from the pool since it is usually introduced via swimmers (and their swimsuits) from lakes and other bodies of water where the algae persists.  Unlike green algae, it does not usually blow in through the air.  To get rid of an existing outbreak, one must shock to higher levels with an FC that is 60% of the CYA level.  One must get behind light niches, under removable ladders, and must put in pool equipment (poles, brushes, skimming nets, etc.) and should wash swimsuits.  Remember that this algae likes shade so will often hang out in these often ignored areas.
  • Pink Slime.  This is actually a bacteria and is usually easily killed by chlorine and prevented at the same chlorine levels as used to prevent green algae.  Shock levels to kill existing pink slime are the same as that for green algae.  It is far more commonly seen in Baquacil/biguanide/PHMB pools.
  • White Water Mold.  This looks like white tissue paper that easily falls apart and is usually easily killed by chlorine and prevented at the same chlorine levels as used to prevent green algae.  Shock levels to kill existing white water mold are the same as that for green algae.  It is far more commonly seen in Baquacil/biguanide/PHMB pools.


The first step in clearing a pool with algae is to physically remove as much debris as possible.  It is a waste of chlorine or other products to try and get rid of leaves and other material that can be more readily removed through physical means.  Some people use ProTeam® System Support to help raise organics to the surface for easier physical removal.  The next step is to shock the pool with a high chlorine level (if algaecides noted below are to be used, these are often added prior to shocking, except be careful of copper as raising the pH can cause staining).  Shocking to kill algae and to clear a pool is completed when the overnight chlorine loss test loses <= 1 ppm FC, when the Combined Chlorine (CC) level is <= 0.5 ppm and when the water is clear.  The key to shocking is maintaining a sustained high chlorine level along with regular brushing and 24/7 circulation with backwashing/cleaning of the filter as appropriate.  Note that some pools have poor circulation so need more regular brushing.  For speedier clearing, some people use clarifiers or flocculants, but these vary in how well they work depending on the type of contamination in the pool (they do tend to work well for algae).


The above discussion talked about how to prevent and kill algae using chlorine alone.  At extra cost, one can also use algaecides and/or phosphate removers which are described below.


  • Linear quats.  These can inhibit many forms of algae and are relatively inexpensive, but can foam so are not suitable for pools that have aeration (i.e. waterfalls, spillovers, etc.).  They also tend to break down more quickly from chlorine so increase chlorine demand.  It is added weekly.
  • Polyquat.  This can inhibit many forms of algae and is more expensive then linear quats, but it does not foam and breaks down more slowly from chlorine.  It is added weekly.  Polyquat is also a clarifier, though not as powerful as specialized clarifiers.
  • Borates.  50 ppm borates, usually added in the form of boric acid (though can also be added inexpensively from a combination of 20 Mule Team Borax and Muriatic Acid) are a mild algaecide and a pH buffer.  It is only removed from the water by dilution.
  • Copper.  This is a very effective algaecide but unfortunately the level that is most effective is near the level at which staining occurs unless the pH is kept low.  Copper ions can also turn blond hair green.  It is only removed from the water by dilution.
  • Sodium Bromide.  This turns the pool into a bromine pool.  It works well in pools with high CYA levels where it would take extraordinarily high FC levels to kill algae.  Since bromine is not affected by CYA, it is able to kill algae.  If not overdosed, the bromine will eventually break down to bromate (a suspected carcinogen in drinking water) from sunlight or will outgas.
  • Ammonium Sulfate.  When an equal amount of chlorine is present, it produces monochloramine.  It works well in pools with high CYA levels where it would take extraordinarily high FC levels to kill algae.  Since monochloramine is not affected by CYA, it is able to kill algae.  The monochloramine can be oxidized by addition of more chlorine.
  • Phosphate removers.  This is lanthanum chloride (or sometimes lanthanum carbonate) that precipitates lanthanum phosphate.  It initially clouds the water forming lanhanum carbonate and lanthanum phosphate, then gets caught in the filter and captures more phosphate.  Some products contain a clarifier to remove the cloudiness.  Phosphate removers slow down algae growth by removing orthophosphate, but do not remove organic phosphates.


The only one of the aforementioned algaecides that is guaranteed to prevent algae even with zero chlorine levels is copper, but it would need to be at levels that would stain unless the pH is low.  All the others significantly slow down algae growth (the bromide/bromine and ammonium/monochloramine kill algae, but are not permanent) and may appear to prevent it completely depending on growth conditions.  For pool services, weekly Polyquat, one-time borates, and maintenance-level phosphate removers are the most practical choices with the least side effects.  Nevertheless, even these can fail if the active chlorine  level gets low enough.  For Polyquat that failure may occur around an FC that is 1.0-1.5% of the CYA level.  Borates may be similar.  Phosphate remover failure depends on the level or organic phosphates where at zero chlorine levels bacteria can rapidly convert organic phosphate to orthophosphate for algae to use.


For pools with an existing algae outbreak and high CYA levels where one cannot do a partial drain/refill to lower that level, the use of ammonium sulfate is a reasonable "get in, get out" approach since the monochoramine is temporary and can be readily removed by additional chlorine.  Though monochloramine kills bacteria slowly, it is reasonably effective against algae.

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Replies to This Discussion

Nice!  I will be following this one very closely, as we get an awful lot of calls on algae.


Thank you, Richard!



Good write-up on "algae" Richard.

I would to like to offer the following formula for those who want to add borax for algae control.  20 Mule Team Borax is usually sold as sodium tetraborate decahydrate (10 Mol water).  If I have my numbers right, the equation is as follows: Volume of pool water, divided by 13,600 (formula number for borax 10 mol), times 50 (ppm of boron suggested in Richard's write-up) equals pounds of borax to add.  For example, 20,000 gallons of water, divided by 13,600, times 50, equals 73.5 pounds of borax to add.

Some chemical companies sell a stronger strength of borax, which is sodium tetraborate pentahydrate (5 mol water).  If adding that strength, then substitute 13,600 (the formula number for 10 mol) with 17,800 (for formula number for 5 mol).

I personally think that it would be better to add that amount of borax over a period of several weeks.


The following are amounts needed to get to 50 ppm Borates (technically ppm Boron) per 10,000 gallons:


36.8 pounds (7-3/4 76-ounce boxes) of 20 Mule Team Borax (sodium tetraborate decahydrate) plus 282 fluid ounces (2.2 gallons) full-strength Muriatic Acid (31.45% Hydrochloric Acid)


28.1 pounds of Proteam Supreme (sodium tetraborate pentahydrate) plus 282 fluid ounces (2.2 gallons) full-strength Muriatic Acid (31.45% Hydrochloric Acid)


23.9 pounds of Proteam Supreme Plus (actually of boric acid).  pH will only slightly drop, perhaps 0.2 units.  You can get boric acid from The Chemistry Store or AAA Chemicals.


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