Tips and Tricks for Identifying and Treating Tough Algae

Some pool algae are easy to treat and remove; however, other types can be quite challenging to deal with.

As the weather warms up and the swim season starts, pool water quality begins to suffer and algae often start to grow. Some algae are easy to treat and remove; however, other types can be quite challenging to deal with. The three main types are green, black, and yellow/mustard algae. Green algae is relatively easy to eradicate, but yellow/mustard and black algae are difficult to kill and, as a result, can make the job of a pool service professional difficult.

Green algae

The growth and spread of this algae is usually due to a lack of sanitizer and or circulation in the pool. When it is present, this algae can typically be identified by the slight cloudiness of the pool water. As it continues to grow and thrive, the water quality gets worse and, in extreme cases, it eventually deteriorates to the point where the pool steps and floor become difficult to see.

Green algae should be dealt with quickly to prevent these water clarity problems, as cloudy pool water can make it difficult to see someone who may be having trouble in the water and, thus, increase the risk of drowning.

Green algae spores are common and are typically found when chlorine levels are not maintained during especially warm, sunny periods. This algae can also be introduced as a result of cross-contamination when toys or swimsuits that have been used in a river or lake are then used in the pool. Technicians using brushes and poles to clean algae, but neglect to clean them before servicing the next pool, could, in fact, cause outbreaks in the others they service. That said, green algae is by far the easiest to prevent and kill. High doses of chlorine usually eradicate it quickly; the process can also be accelerated with the addition of a good algaecide, which will also make the customer happier much faster.

Once the water has been treated to kill the algae, the pump and filter have the burden of clearing the water. The filtration system must continually run to trap the dead algae and clear the water. A water clarifier can also be used to help expedite this process. When the pool’s water is properly balanced and maintained (using chemicals and sanitizers) it is fairly rare for green algae to grow.

Black algae

This 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 where calcium deposits have developed. These areas act as small foxholes where water does not circulate well, making it an easy area for black algae to grow and thrive.

Black algae is a problem that is almost exclusive to gunite pools. It is tough to treat because it burrows into the plaster and forms a protective layer, making standard chemical treatments ineffective. As this algae needs a place to grab onto, it becomes an even bigger problem in plaster pools where the surface has deteriorated. Cracks and rough areas around light fixtures, ladder rails, or broken tiles are prime habitats for black algae. It also tends to be more prevalent in rural, dry agricultural regions, as it can be introduced into the pool via debris in the air.

This algae should be treated as soon as it appears, as the bigger the head of the algae spot the deeper it burrows into the plaster and the harder it is to kill. There are several ways to treat it, including scrubbing the affected area(s) with a stainless steel brush for an extended period of time to remove its protective coating. Brushing should be repeated daily until the algae is eradicated from the pool. In severe cases, the spread of the algae may be controlled; however, it may not be eliminated entirely.

Yellow/mustard algae in a blue pool will make the water look green, so it is important to ask the right questions and be sure about any observations before making a diagnosis or treating the water.

Yellow/mustard algae in a blue pool will make the water look green, so it is important to ask the right questions and be sure about any observations before making a diagnosis or treating the water.

After brushing the area(s), the next step is to chemically treat the surfaces, as well as the water, to rid the pool of the algae. One of the most effective treatments involves using granular trichlor on the pool’s horizontal surfaces and a copper-based broad spectrum algaecide for the vertical surfaces, in addition to the pool water. This process first involves a trichlor treatment then, after the chlorine level recovers and the water is balanced, a copper-based algaecide is applied.

Broad spectrum chelated-copper algaecide can also be used as a preventive measure. Some copper algaecides are triple chelated and will protect the pool for months at a time, giving a service technician the ability to effectively brush and remove 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 is not any non-chelated metals from the pool equipment or source water present. If metals are in the pool water prior to the addition of a metallic algaecide, the water can become oversaturated, which causes 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 algae heads that have been brushed off to remove them from the system. Brushing is important to control this algae because of its ability to form a protective layer, making it resistant to chemical treatment. Further, any remnants of the black algae that remains in the pool can regrow into new problem areas; therefore, to keep it under control, it is important to maintain a higher parts per million (ppm) of chlorine along with using a broad spectrum copper-based algaecide.

Yellow/mustard algae

Yellow/mustard algae may be one of the most difficult a service technician may face. One reason for this is because it is often misdiagnosed—sometimes it is mistaken as green algae and in other cases as pollen, or just dirty pool water.

Yellow/mustard algae in a blue pool will make the water look green, so it is important to ask the right questions and be sure about any observations before making a diagnosis. To eliminate the possibility of misdiagnosing yellow/mustard algae as green algae, the first question to ask the homeowner is if their pool water is cloudy. If the answer is “no,” then it is likely not green algae as it makes the pool water cloudy and murky.

The factors of algae growth
• Photosynthesis (sunlight): Algae is a plant and like plants it is able to take light and generate its own food source for growth.
• Organic materials: Leaves, barks, seeds, and grass clippings can introduce algae into the pool water.
• 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 of iron in water to an increase in the intake of carbon which makes algae grow more rapidly.
• Lack of chemical maintenance: Zero chlorine in the pool water combined with a hot summer day may result in 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 of algae growth—even in chemically maintained pools.

To rule out pollen or dirt, the next question to ask the homeowner is if the ‘dirt’ returns right after brushing the pool walls. If the answer is “yes,” it is not dirt/pollen as this debris would simply drop to the bottom of the pool and would not cling to the walls. Therefore, if the pool water is not cloudy and the ‘dirt’ is clinging to the walls after brushing, the problem is likely yellow/mustard algae.

This algae is quite resistant to high chlorine levels and will grow and thrive in a well-balanced chemically treated pool. In fact, the pool’s maintenance can be by the book, but in the case of yellow/mustard algae it will still grow. It is often introduced to pools via lakes, ponds, wind, rain, and even skimmers/leaf rakes, vacuum hoses/heads used in other pools which are diagnosed with this algae.

Although yellow/mustard algae can be easily brushed off of the pool’s surface, it will return in the same place a few days later—especially in shady parts of the pool. When treating this algae it is important to use an algaecide or chlorine enhancer that specifically targets yellow/mustard algae. Sodium bromide will eradicate this algae rather quickly; however, it is only good for a one-time kill. In other words, it will not prevent this algae from returning the next time it rains. That said, for long-term protection, use a chelated and a broad spectrum copper-based algaecide to help prevent it from returning.

The importance of routine maintenance

Regular maintenance helps prevent algae growth; therefore, not only should the water be chemically maintained, the pool should also be physically cleaned (e.g. brushing, vacuuming, cleaning skimmer/pump baskets, etc.) to make it less likely for algae to bloom. It is also important to make sure the pump(s) and filter(s) are operating at least eight to 10 hours a day in the summer. Algae treatments are only effective if the pool water is balanced and is being filtered and circulated properly. Further, it is important to ensure the pool’s filters are cleaned just prior to fighting an algae problem.

When dealing with algae in an older pool with rough plaster, the pool should be resurfaced or, at the very least, any rough areas should be sanded smooth.


This article was written by Terry Arko and originally appeared on Pool & Spa Marketing [link].