Tips and Tricks For Identifying And Treating Tough Algae

By Terry Arko

As the weather warms up and the swim season starts, swimming pool water quality beings to suffer and algae often begins to grow.  Some algae is easy to treat and remove but other types of algae can be very challenging to deal with.  There are three main types of algae, Green algae, Black algae and Yellow/Mustard algae.  Green algae is pretty easy to eradicate, but the mustard and black algae is difficult to kill and can make the job of a pool service professional challenging.

Green Algae:  Green algae is the type of algae that suspends itself in the water making pool water murky.  In extreme cases, green algae can get so thick it can even make it difficult to see the steps of the pool!   The growth and spread of green algae is usual due to lack of sanitizer and or circulation in the swimming pool. It will start as a slight cloudiness in the pool and as the algae continues to grow and thrive the water, the water quality gets worse and eventually deteriorates to where you can’t see the bottom of the pool. This type of algae must be dealt with quickly to solve the water clarity issue. Cloudy pool water is very dangerous due to the increased risk of drowning. Green algae spores are very common and this is usually the form that will be seen when chlorine levels are not maintained during especially warm and sunny times. Green algae can also be introduced as a result of cross contamination when someone brings toys or even swim suits that have been in a river or lake and then are introduced in the pool. Pool Pros who use brushes and poles to clean up algae pools but neglect to clean the brushes before servicing the next pool could see an outbreak in the pools they service. However green algae is by far the easiest algae to prevent and kill.  High doses of chlorine usually quickly kill the algae and the process can be accelerated by the addition of a good algaecide—to make your customer happier, faster.   Once you have treated the pool to kill the algae, your pump and filter have the burden of clearing your water. You must continually run your filtration system in order to trap the dead algae and clear the water.  You can also use a water clarifier to help expedite the clearing process. Maintaining the proper water balance with chemicals and sanitizers as well as properly maintaining the pool’s equipment, it’s fairly rare to grow green algae.

Black Algae:  Black algae will generally appear as black dots on pool plaster, usually in the pitted area of the pool where the plaster has been etched or maybe where some calcium deposits have developed. These areas act as small fox-holes where the water doesn’t circulate well, making it an easy area for Black Algae to grow and thrive.  Black Algae is almost exclusively an issue in gunite pools and it is tough to treat this type of algae because it burrows into the plaster and forms a protective layer over itself making the standard chemical treatments ineffective.  Black algae tends to be more prevalent in rural, dry regions where there is plenty of agriculture and farm animals.  It can be introduced into the pool in the dust that blows about in these regions. However, Black algae needs a place to grab onto and so it becomes a bigger problem in plaster pools where the surface has deteriorated and is rough. Cracks and rough areas around the light or ladder rails is a common place to see black algae begin to appear as well as around broken tile which are prime habitats for black algae.

It is important to start treating the black algae as soon as it appears because the bigger the head of the algae spot the deeper it is buried into the plaster and the harder it will be kill.  There are several ways to treat the black algae once it appears and all of the treatments include brushing the affected areas for an extended period of time in order to remove the protective coating the black algae uses as a defense against chemicals. The first step to treating black algae is to brush the affected areas with a stainless steel bristle brush in order to remove the protective layer so the algae treatment will be effective. You then must continually brush the affected areas daily until you rid your pool of the algae. If it is a severe case, you may only be able to control the spread of the algae, but not totally get rid of it.

After the black algae has been brushed, the next step is the chemical treatment to eradicate the algae.  One of the most effective treatments involves using granular trichlor on the horizontal surfaces and a copper based broad-spectrum algaecide for the vertical surfaces and throughout the overall pool water.  The process involves treating with the trichlor first and once the chorine level recovers and the water is balanced, apply the copper based algaecide. The use a broad-spectrum chelated copper algaecide can also be used as a preventive. Some of these copper algaecides are triple chelated and will protect for months at a time, giving the service professional the ability to effectively brush and remove the black algae, layer by layer.  It is always a good idea to have the water tested for metals prior to the addition of any metallic algaecide to be sure there aren’t any non-chelated metals from the pool equipment or source water.  If you have metals in your pool water prior to the addition of a metallic algaecide you could over saturate the water and cause problems such as staining and or water discoloration.

In the case of black algae it is extremely important to always vacuum or brush and backwash /clean any remnants of the black algae heads that have brushed off in order to remove them from the system Brushing is key to controlling black algae because of the black algae’s ability to form a protective layer over itself making it very resistant to chemistry.  Any remnants of the Black algae that remain in the pool can re-grow into new problem areas.   In order keep the black algae under control, it’s important to maintain a higher ppm of chorine and keep the phosphates below 200 ppb along with the addition of a broad spectrum copper based algaecide. Brush, brush, and brush the pool and wash those brushes before putting them into a new pool!

Yellow/ Mustard Algae:  Mustard algae the most difficult algae for a number of reasons. The biggest problem with Yellow/ Mustard algae, is that it is often misdiagnosed. Yellow/ Mustard algae is often mistaken as green algae or sometimes as pollen or sometimes it just makes the pool look ‘dirty.’   If you think about it, yellow algae in a blue pool will make the water look green (yellow + blue = green).  So it’s important to ask the right questions and be careful about the observations you make when diagnosing the algae.  Asking the correct questions is key to proper diagnosis. Since yellow/ mustard algae is often misdiagnosed as green algae so it is important to eliminate this possibility.  The question to ask:  Is the pool water cloudy?   If the answer is NO, then it’s probably NOT green algae because green algae makes the pool water look cloudy and murky.   To determine if what is being observed is pollen or dirt or yellow/mustard algae, the question becomes:  After the walls have been brushed, does the “dirt” come right back?  If the answer is YES, then it’s not dirt/ pollen—as dirt/ pollen would simply drop to the bottom of the pool and would not cling to the walls.  So if the pool water isn’t cloudy and the ‘dirt’ is clinging to the walls after brushing, the problem is likely be yellow/mustard algae.

Yellow/ Mustard algae is very resistant to even high chlorine levels and will grow and thrive in a chemically well-balanced pool. Let’s face it, we all associate algae growth with something we have done wrong like not running the pump, running out of chlorine, clogged baskets, etc…but in the case of yellow/mustard algae, the maintenance of the pool could be “by the book” and yellow/mustard algae will still grow.  Yellow/ mustard algae is often introduced to swimming pools from lakes, ponds, wind, rain and even skimmers/leaf rakes, vacuum hoses & heads from other people’s pools with mustard algae.  Yellow/ Mustard algae tends to be more prevalent near the warmer waters of the southern United States, but still does show up on occasion shows up in the NE and Midwest.

Yellow/ Mustard algae tends to brush off easily from the surface, but will show back up in the same place a few days later.   This often happens in the shady part of the pool.  To treat this type of algae it’s important to choose an algaecide or chlorine enhancer that specifically targets the yellow/mustard  algae.  Sodium Bromide has proven to be an way to kill yellow/ mustard with excellent, fast results, but it is only good for the one time kill. It won’t keep you from getting yellow/mustard algae the next time it rains. The best recipe to prevent yellow/ mustard algae from coming back is to control the phosphate levels in the pool and keep them under 200ppb and using a chelated, broad-spectrum, copper based algaecide for long term protection.

Regular maintenance does help prevent algae growth so it’s important to stress that the pools should always be maintained both chemically and physically by brushing, vacuuming, cleaning skimmer/pump baskets etc to make algae less likely to bloom.  Keep chemical levels in balance and always be sure pump and filters are all running at least 8-10 hours a day in the summer.  Algae treatments are only effective if the swimming pool and spa water are balanced and are being filtered and circulated properly.  Always be sure that the pool and spa filters are cleaned JUST PRIOR to fighting an algae problem. Also, older pools with rough plaster should be dealt with by resurfacing or at the least sanding and smoothing rough areas.  And always test the phosphate levels in the pool prior to treatment as well. Rather than waiting until later in the summer to test for phosphate levels, after several algae booms, it’s best to test early in the season and be sure the phosphate level never exceeds 200ppb.  


•       Photosynthesis –Sunlight-Algae is a plant and like plants it is able to take light and generate its own food source for growth

•       Phosphates, Carbon and Nitrates- These are the prime nutrients algae needs for strong growth. The simple removal and lowering of one of those nutrients phosphate can go a long way in keeping algae from blooming.

•       Organic materials- Leaves, barks, seeds and grass clippings all can bring algae into the pool

•       Rough surfaces- Algae loves to have something to cling to on the surface. It is very difficult for algae to get a foothold on smooth surfaces.

•       High iron content- Some studies have linked the presence iron in water to an increase in the intake of carbon which makes algae grow more rapidly

•       Lack of chemical maintenance – Zero chlorine combined with a hot summer day can mean algae within as little as 24 hours.

•       Poor filtration or circulation – Water that is moving and being cleaned of particulate debris is much less likely to produce algae. Poor filtration and little to no water movement are two of the biggest causes for algae growth even in chemically maintained pools.

Terry Arko has more than 30 years of experience in the swimming pool and spa industry, working in service, repair, retail sales, chemical manufacturing, customer service, sales, and product development. A certified pool operator (CPO) and CPO instructor through the National Swimming Pool Foundation (NSPF), Arko is currently a water specialist for SeaKlear, which recently joined the NC Brands (formerly Natural Chemistry) family of pool and spa products.  He can be reached via e-mail at