Did the word ‘algae’ catch your attention? It’s normal for all pool owners to worry about algae in their fibreglass pools. Once they appear, algae can be a major problem, especially during the summer months.

If you always thought all algae were the same, you were mistaken. Algae are one-celled plants and are found in more than twenty thousand varieties. The most common algae found in swimming pools are green algae, black algae, pink algae, and yellow algae. Although green algae are the most common in freshwater, they are easier to get rid of. You have a problem in hand if you notice black algae in your pool.

For fibreglass pool owners it is not always easy to recognise which kind of algae is growing in their pool. Knowing how to identify black algae and ways of getting rid of them can help them take immediate action without wasting any time.

What are black algae?

Black algae or cyanobacteria (photosynthetic bacteria) are not black! It is a blue-green alga that appears black due to the presence of chlorophyll and water-soluble pigments. These are mostly found in freshwater bodies like lakes and ponds. However, they can find a home in unmaintained pools where the water circulation and cleaning are not proper.

How to know if there are black algae in your pool?

It is easy to miss out on the signs of black algae in your fibreglass swimming pool perth when they first appear. Due to the microscopic size of algae, it is not possible to identify a single alga with the naked eye.

It’s only when millions of these tiny plants have accumulated on a spot they become noticeable. It is vital to spot them as soon as possible. Once a colony of black algae forms on your pool surface, they grow rapidly and begin to anchor itself on the surface.

The roots dig into porous surfaces and spread everywhere in the pool and concrete joints. Even when you take action to destroy them and think that you have been successful in your mission, to your surprise they may reappear soon. Elimination of the visible portion of these algae does not mean the entire colony has been successfully destroyed.

Keep a close watch on your pool. If you notice these things, you know it’s black algae you are dealing with.

  • Black spots with raised heads are appearing on the pool walls and the floor
  • The algae growth is mostly in rougher areas of the pool or the plaster
  • It is difficult to brush them from the pool surface
  • The black spots/patches are multiplying rapidly
  • Even after you scrape them off, they reappear soon
  • There is a slimy layer on top of the black spots

How did I get black algae in my pool?

No, it’s not always your fault that you have black algae in your pool. Even with the utmost pool care and maintenance, black algae can invade your fibreglass pool. If you’re wondering, with your filtration and sanitation systems in place, how can algae grow in your pool, the reasons are many.

  • Black algae can enter your pool when swimmers bathe in lakes and ponds and enter the pool wearing the same swimsuit.
  • If swimmers enter the pool without proper washing.
  • It is rare, but black algae can also enter your pool through airborne spores.

Are black algae harmful?

Black algae in your pool can not only make the water look murky, alter the taste, and make the water smelly but also lead to health hazards. The rapidly growing, unsightly black algae produce cyanotoxins, as they bloom. These toxins are poisonous and can have various effects on human and animal health.

Swimming in and drinking water with cyanotoxins may cause health conditions like –

  • Headaches
  • Nausea
  • Vomiting
  • Diarrhoea
  • Stomach cramps
  • Liver damage
  • Neurological symptoms such as muscle weakness
  • Skin irritation/itching/rashes
  • Redness and soreness in the eyes

In case you notice greenish, bluish, dark foam, or mats on your fibreglass swimming pool surface, manage the bloom before allowing swimmers inside the pool. The water can become extremely toxic if the bloom is severe.

It is not always necessary to drain out the entire pool water to fight and eliminate black algae. Various treatments are available for clearing up these growths and restoring the water quality. However, the best is to drain your pool water completely and give it a deep clean before filling it up with fresh water.

How to get rid of black algae from your fibreglass pool?

Prevention is better than cure. If you monitor a few things carefully, like not allowing swimmers to enter the pool without washing properly, the risk of black algae can be greatly reduced. Vacuum your pool regularly, maintain the pool chemistry, and shock your pool to keep it sanitised.

However, unfortunately, if stubborn-to-remove, black algae have already nested in your pool, here’s what you need to do to get rid of them.

  • Scrub your pool surface thoroughly to remove the slimy protective layer of the algae
  • Scrubbing is important before any treatment to break open the slime layer. Without that, the treatment won’t be effective
  • Scrub all pool accessories well. Any remains of the black algae will soon lead to regrowth and a bloom
  • Scrub toys, ladders, flotation devices, and anything else that often gets wet and stays moist around the pool
  • Wash all swimwear after bathing anywhere outside your pool
  • Apply pool treatment. Shock your pool. Maintain the pH at 7.4-7.6. Create an alkalinity level of 120 – 150 ppm to kill the loose fungal spores
  • Make sure pool users wash properly before entering the pool
  • Run the pool filter to circulate the water and remove the dead algae cells and other particles in the water.
  • Continue the routine at regular intervals and be vigilant. Run the pool filter daily to remove all the remains of black algae.


For pool owners, treating black algae can be a challenging task. In case of a boom, fibreglass pools are easier to remove compared to the other kinds of pools, due to the smooth, non-porous surface. It can be scary to notice black spots in your pool. The bright side is, with the right products, and pool care routine, black algae can be cleaned up and the pool water quality can be restored.

Jackson Martin

Jackson Martin, holding a degree in Civil Engineering from the University of Texas, has been a prominent figure in the home improvement sector for over 18 years. He joined our team in 2020, sharing his expertise in renovation techniques, sustainable building practices, and DIY projects. Jackson's previous experience includes working in residential construction management and as a home renovation consultant. He is an advocate for energy-efficient homes and enjoys participating in local marathon events.

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