What is Black Spot?

Often appearing dark green or almost blue, the term ‘Black Spot’ has been given to this particular algae as it features a dark spot on the top of the growth. This is usually the only visible part of the algae which tends to grow in the harder to reach areas of your swimming pool. It also likes very shady parts of your pool and can be hiding in your pipework, pump or filtration system.

If you only remove the head of the black spot, the algae will return within just a few days as it has many layers that protect it from chlorine. Black Spot is regarded as a slow-growing form of algae, but once established it is very challenging to get rid of. The spores can survive on the pool’s surfaces even when the pool has been emptied and will continue to grow again once the pool has been refilled.

What causes Black Spot?

Black Spot is typically caused by poor chlorination or high phosphate levels. The algae can feed on the phosphates, stimulating its growth. However, Black Spot has been known to survive even when there are low phosphate levels in the pool water. The key is keeping the water balanced with adequate chlorination.

Other factors that can encourage the growth of Black Spot include:

  • Consistently high pH
  • Consistently low cholerine levels
  • Old and worn pool interiors
  • Foliage near pool

How do I prevent Black Spot?

As Black Spot is notoriously difficult to remove from your swimming pool, working to prevent it from occurring in the first place is better than a cure.

If you have a chlorinated pool, keeping the free chlorine levels above 1.5 parts per million is a great start in preventing the growth of Black Spot or any other algae.

It’s also important to ensure the chlorine is distributed evenly throughout your pool and spa. This can be achieved by keeping the water circulating adequately. Running your pump for 8 to 12 hours a day, all season long is recommended along with keeping your pH between 7.4 – 7.6 and alkalinity levels between 120 – 150 ppm at all times.

Keeping your pool clean by regularly removing debris and vacuuming the entire pool is essential if you wish to prevent algae growth. Don’t forget to clean the additional pool equipment including ladders, steps, diving boards, slides, pool toys and the pool cover.

Tips to remove Black Spot

Black Spot
keeping your pool clean is have the battle to preventing Black Spot

Removing Black Spot can be time-consuming. Depending on the extent of the algae growth, it may pay to hire a professional team to get rid of it once and for all. If you’re unsure of whether you have Black Spot, you can have your water tested which can help guide you towards the most effective treatment.

Alternatively, you can follow these steps.

1. Clean the equipment

Using a specialised cleaner, sanitise all items that enter your pool including the equipment, pool toys, floats and even bathing suits.

2. Brush your pool

Using a bristled brush methodically clean all the surfaces of your pool, including the difficult corners and around fixtures. You’ll need to brush hard as you’ll have to break down the Black Spot membrane and remove the spores to stop further growth.

If your pool is concrete, use a stainless steel bristle brush. If your pool has a plaster surface, use the softer nylon bristle brush. This is the most time-consuming step, but the most necessary.

3. Clean your filter

If you have Black Spot growing in your pool, it’s highly likely you’ll have it in your filter too. If your pool uses a DE or sand filter, perform a backwash and rinse your filter further a couple of times. For those with a cartridge filter, remove the cartridge and thoroughly rinse it off.

4. Check your water chemistry

Check you have adequate chlorine in the water for the size of your pool. Ensure the pH levels are ranged between 7.4 – 7.6 and that your alkalinity levels are between 120 – 150 ppm.

5. Shock your pool

Next step is to shock your pool, but you’ll want to use three times the normal dosage. Do this at night and run your filter system for 24 hours until the Black Spot has been removed.

6. Brush your pool again

After shocking your pool, brush it again while your chlorine level remains high. Some experts recommend you continue brushing for several days until you feel confident that the Black Spot has gone.

7. Shock your pool again

Shock your pool again after a few days since your previous shock. This time, do so at the normal amount.

8. Vacuum & Clean

Vacuum your pool to remove any debris that may have surfaced after your brushing efforts. Clean your filter again to remove any Black Spot residue.

It’s important to note, your chlorine levels may remain high for up to two weeks. Continue to maintain your pool’s chemistry and test before you dive in.


For more advice on maintaining your pool, visit the pool section of our Learning Library.

Leading image source: cleanpools.com.au