Black spots in your swimming pool can be caused by a variety of factors including Black Spot Algae, Metal Staining, or Osmosis in the case of fibreglass pools. If you have found black dots in your swimming pool below are a few steps you can take to determine the cause and the correct course of action.

1. Test Your Water Chemistry

Black spots will only ever appear if one or more of your water chemistry parameters is out of range. If your results show that your PH is high and your Chlorine level is low it is likely that the black dots are caused by Black Spot Algae. If your water chemistry reading indicates a high level of copper coupled with high PH then you are more than likely looking at metal staining. 

This article will discuss the removal of Black Spot Algae. Metal staining as well as osmosis in fibreglass pools will be discussed in a separate article.

Testing Pool PH

2. Black Spot Algae

If your testing determines that you have black spot algae you will need to follow the below procedure;

1. Taking into account that black spot requires a porous surface to attach itself to, you will rarely find this condition in a fibreglass pool, if you do you will want to ignore my instructions below to use a wire brush, this will scratch the delicate gel coat in your fibreglass pool.

2. Thoroughly brush all affected surfaces with a wire brush, you need to apply a lot of force here as the aim is to crack open the cell membrane in order to allow the chemical treatments to penetrate the cell walls in order to get rid of the black spot algae for good.

3. Perform a backwash and rinse of your media filter, or thoroughly clean you cartridge filter.

4. Next you will need to drop the PH level to 7.0, you can use Hydrochloric acid, or Sodium Metabisulphite in order to achieve this. Dosage levels can be determined by using an online calculator provided you know what your pool volume and the current PH level is. The lower PH level is required as it will soften the waxy coating surrounding the Algae cells allowing for easier penetration by the following chemical treatments.

5. If phosphate levels are high, treat with the appropriate dosage of Lo-Chlor Starver M, or Starver X.

6. Next you will add 1L of Lo-Chlor Black Spot Algae Killer per 50,000L of pool water and set the filter to run for 24 hours non stop. When added to a pool with a salt/mineral chlorinator this product will occasionally react with the Chlorine gas being produced in the cell causing a layer of fine bubbles on the surface of your pool, this is nothing to worry about and generally clears up after 48-72 hours.

7. After the system has been running for 24 hours perform another backwash of your media filter, or thorough clean of your cartridge filter note that it is important to use a filter cleaning chemical for cartridge filter in order to remove as much remnants of algae in the filter as possible. It is sometimes best to replace the filter element if it is heavily infested.

8. Run the system for a further 24 hours straight, and then repeat the filter cleaning process.

9. At this point you should have noticed a decent improvement in the appearance of the black spot, the job is not over yet though. You will need to retest the water chemistry adjusting the PH back to 7.0 if necessary, and thoroughly brush the walls and floor of the pool with your wire brush.

10. Next you will need to apply a shock dosage of Chlorine to the pool in order to finish off the job, retest at regular intervals over the next 3 days and maintain the pool at shock level for 72 hours.

11. Repeat the filter cleaning process at this stage and reassess the problem, in most cases the Algae should be done and dusted, for heavily infested pools you may need to maintain the pool at shock level for a further few days.

Swimming Pool PH Chart

3. Regularly Test Your Pool Water

Now that the black spot is taken care of you will want to make sure it doesn’t return, this means you will need to regularly test the pool water and ensure Chlorine levels are maintained within the recommended range at all times. Keeping the pool clean and free of debris is also important.

Testing your pool water

4. Filter Your Pool Water

You also need to ensure adequate water turnover to keep the black spot at bay, in most cases this would be somewhere between 8-10 hours of filtration runtime per day during the warmer months, and 4-6 hours in the winter months. Larger pools may require longer running hours.

Clean your swimming pool filter often

Prevention is Always Cheaper. Be proactive!

Prevention is always cheaper than the cure which is why Clarity Pool Management Sydney recommends that you have a regular maintenance plan for your swimming pool. Our pool service technicians can attend to your Sydney swimming pool in person once a month ensuring you never have to worry about this and other issues, alternatively you will need to find a trusted pool shop to have your water tested by a professional on a monthly basis at a minimum.

Get in touch with our team today for advice on the best maintenance plan for your Sydney swimming pool.

Summary of Key Points

  • Black spots in a swimming pool can be caused by Black Spot Algae, Metal Staining, or Osmosis.
  • Testing the water chemistry is crucial to determine the cause of the black spots.
  • If Black Spot Algae is identified, a specific procedure must be followed, including brushing the affected surfaces, dropping the pH level, using appropriate chemical treatments, and performing regular filter cleaning.
  • Regular testing, maintenance, and filtration are essential to prevent black spots from returning.
    Clarity Pool Management Sydney recommends having a regular maintenance plan for the pool to prevent issues, and their pool service technicians can attend to your Sydney swimming pool once a month to ensure proper maintenance.