Black Spots in pool

Black Spot Algae

Black spot algae on pool surfaces is an algae issue that some pool owners battled with for several years. This is because this form of algae is very resilient to normal levels of chlorine and brushing. It forms a tough biofilm over itself which protects it from the chlorine in the pool.  Because of this protective layer, it is far more resilient and stubborn than normal green algae.

Even once you remove the spots, they often return within a few months and start spreading in the pool once again.  This is why it is very important to follow the right advice in removing it properly the first time.  Don’t worry this page will help you get rid of it and keep it gone.

Shock chlorination, regular brushing, and a quality algaecide are sometimes successful approaches to removing black spot algae. If that is not working for you then other suggestions on how you can remove black spots are here. Remember, whatever solution you try, make sure that it treats all of the pool water everywhere including the pipework and filter, not just in the main pool. Otherwise, the algae will be back within two weeks.  Contact us for more information.  Pictures supplied by:

Black Spot algae

How to remove black spot algae

It is important when following these directions that you use top-quality products.  This is because cheap products do not have the strength to do the job properly and can stain the pool.  In short, if you are planning to do this on the cheap, don’t waste your time and money because it will not work.

  1. Vacuum and firmly brush the entire pool surface, particularly where the black spot is present.
  2. Test the pool water and phosphate levels at your local pool shop.  Add what is needed to balance the water within the normal ranges suggested by the pool shop.
  3.  Add 2 kgs of granular stabilized chlorine to the pool water and run the pump for 15 minutes while brushing the spots again.
  4. Add a high-quality copper-based algaecide to the pool.  Then dissolve it in a bucket of water first before adding it to the pool.
  5. Add a bottle of ployquat algaecide to the pool at the same time as the copper algaecide.  These products will work together to penetrate the algae and kill it off.
  6. Run the pool pump for 4 hours after these steps to make sure all the products are mixed into the pool water.

If you don’t have time to gather all of these separately, these items are sold in kit form along with technical support here.  PLEASE Make sure you use top-quality products when following these steps as cheap copper algaecides will stain the pool surface.

WARNING: Don’t acid wash your pool to remove black spot algae

While there are various ways to removing algae, we don’t suggest acid washing the pool. This is because black spot also lives in the pump, filter, and pipes. So even if acid washing removes it off the surface, once you refill it, the algae will simply blow back into the pool and start regrowing.  Acid washing is a waste of money and water.

vblack spot algae in swimming pool
Algae staining in swimming pool
black spot algae in swimming pool

Top 5 causes this forms in your pool

There are several theories and ideas on what causes and how to remove black spot algae. These include:

  1. Surrounding vegetation carrying black spots can fall into the pool and then this algae spreads in the pool. Also, a quick google search of ‘black spot on leaves and plants’ will reveal millions of websites discussing this issue.  Black spot is very common in the garden.
  2. Poor water maintenance will often result in the formation of black spots.  Low chlorine levels and an unbalanced pH are the main contributors to algae getting started in your pool.  
  3. High phosphate levels: Phosphates are essential nutrients for algae growth. So, High phosphate levels in the pool water will make it easier for black spots to grow. Also, the amount of available phosphate is the primary factor controlling algae growth. Zero phosphates are ideal.
  4. Older pebble pools as they age will become more coarse to the touch.  This is because the cement that holds the pebbles together will erode away over time with pool water exposure.  This then creates deeper gaps between the pebbles that become a great hiding place for black spot algae to start growing and spreading.  
  5. Seawater. Some suggest it originates solely from seawater. However, this theory is not very credible as pool owners in Alice Springs experience black spot in their pool if it is not maintained correctly.  

Before and after images of black algae removal

Algae in pool
Black spot algae often starts as green algae.
Algae staining in swimming pool
Green and black spot algae algae lead to surface staining.
Black spot in pools
Black Spot in Pebble Pool

Black spot in fibreglass pools

As a rule, it is very uncommon for black spot algae to form in fibreglass pools.  This is because it cannot attach in fibreglass pools because the surface of the pool is so smooth.  For this reason, black spot is not able to get properly embedded in your pool.  Because of this, it cannot grow in the pool.  Remember, the spots may look exactly like black spot algae but most times it is not an algae that you are seeing but mineral staining. 

In summary, black spots in fibreglass pools are either osmosis or mineral deposits from pool salts or algaecides.  You can remove these spots with a quality stain treatment product.  Remember, an algaecide designed to kill black spots will likely do absolutely nothing at all to remove the spots in a fibreglass pool.

black spots in fibreglass pool

Be Careful: Metal stains often look very similar!

Many times pool technicians or pool owners mistake mineral or copper staining for black spot.  This mistake is usually uncovered when they add granular chlorine to the pool.  Doing this will often make metal stains worse rather than better.  The examples below show copper staining that has been oxidized by granular chlorine.  The images below are NOT black spots.  They have been caused by granular chlorine being added to the pool that has then oxidized the copper on the pool surface and turned it black.  We see this all the time. So if you have dark patches in the pool after adding granular chlorine you have copper staining.  

Do these pools have black algae? (no they don't)

black spot in pools
copper staining in pool
Chlorine free pools with copper staining
Michelle Johnstone
Michelle JohnstoneMiami - Gold Coast
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I had a issue with my new pebblecrete pool that was driving me nuts. These guys were really helpful and directed me to a descaling product that fixed it for me.
Stephen Sedgwick
Stephen SedgwickKellyville - Sydney
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I had a large rust stain on the side of my pool that was growing quickly. It was not until I saw this website that I realised that it was coming from the reinforcing steel. Glad I stopped wasting money trying stain removers.
Stephanie Johanson
Stephanie JohansonToongabbie - Sydney
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I was advised that my pool's interior needed replacement due to staining caused by Black Spot Algae. Thanks to the product sent by Aussie Pools, all the black spot was completely removed in just three weeks. I am extremely satisfied with their solution and so glad I don't need a new pool for no reason.

Final thoughts

Make no mistake, black spot algae removal is no joke.  Some websites even suggest that you can never get rid of it no matter what you do.   We have found that with the right products and process, you can remove black spot algae and keep it gone.  The trick is the use of enough of the right products to kill it off completely and then maintain the water afterward.  Don’t play around with it as it will become resilient to your attempts and become even harder to remove.  

If you are going to treat it, go all the way and do it properly the first time.  Please contact us if you have any questions or are unsure about anything.  We are happy to help you.