How to Fix Stubborn Pool Algae (Mustard and Black Spot)?

Beau Rixon
Algae, once established, can become a constant battle for the pool owner. “Mustard” algae and “black spot” are the most stubborn of algae bloom.

Mustard algae

This variety of algae grows as a yellow and dusty substance and attaches itself to the pool floor, walls or water surface. It isn’t slimy like green algae and can even live outside of the pool, on objects such as pool floats, equipment and bathers. If mustard algae is found on surfaces outside of the pool, disinfect them as soon as possible to prevent it from spreading. Treatment for this algae is relatively easy.

Follow these steps to clear mustard algae:

  1. Disinfect any items that may have been in contact with the algae. As mentioned above, yellow algae can spread outside of the pool on bathers, pool floats and equipment. 
  2. Move any moveable equipment (hoses, poles etc.) to the shallow end and shock the pool. The shock will sanitise the equipment in the pool.  
  3. Pull your sleeves up and start up your vacuum Once the shock has taken effect, you can start to vacuum up the algae (to waste if you have a media filter).  
  4. Re-test your pool chemistry Bring a water sample to Aquafirst so we can ensure your pH and alkalinity levels are balanced and the shock is working effectively.  
  5. Brush your pool using an algae brush If you can still see some algae, brush it so it disperses in the water. This will help the chlorine to kill it.  
  6. Shock your pool again 
  7. Keep brushing We want to be sure we kill any molecule of algae remaining 
  8. If the algae has cleared and seems to stay away for a couple of days, the coast is almost clear. Shock your pool once more for good measure. 
  9. Re-test your water chemistry and re-shock if needed.

How do I prevent mustard algae from growing?

  • Keep your pH, alkalinity and sanitiser levels balanced at all times. Bring water samples in to Aquafirst fortnightly during the swimming season.
  • Run your pump and filter for around 8 hours a day all season long.
  • Vacuum and brush as often as you remember to (just keep thinking of how hard it is to treat algae when you don’t feel like cleaning)
  • Keep your additional pool equipment clean including pool toys, floats, ladders, steps, diving boards, slides and covers.

Black Spot

Black spot algae is a pool owner’s worst nightmare. Once fully established in the grout between the pool tiles or pebbles, stubborn and unsightly algae may result in your pool needing to be drained completely in order to remove it. It’s different from other algae types because it has many layers and roots, so chlorine and normal algaecides cannot kill it. Most commonly, because black algae comes from the ocean it is more often than not transferred by infected bathers or toys. It is important to prevent the further growth of black spot as soon as it is identified. In particular, keep an eye out for dark raised lumps, especially in the corners, around steps and on the water line. If you believe that you have black spot, bring a picture and water sample to Aquafirst. We can provide the products and information you require for treatment.  

After visiting Aquafirst, follow these steps to clear black spot algae:

  1. Brush the algae using a steel-bristle brush (unless you have a vinyl pool) Black algae has an enamel-like coating that makes it hard for chemicals to penetrate and kill it. By using a steel brush, you’re breaking down the tough coating allowing for chemicals to get through and do their job.  
  2. Lower your pH Having low pH levels (7.0-7.2) when treating black spot will allow your algaecide will work more efficiently 
  3. Superchlorinate your pool. By raising chlorine levels your pool will kill black spot algae easier and faster.  
  4. Wait 24 hours and then add a black spot algae killer Do not add block spot algaecide near your skimmer Do not add calcium hypochlorite or any alkaline products 24 hours before or after black spot algaecide
  5. Chlorinate and filter as normal 
  6. Vacuum your pool 
  7. Clean your filter 

How do I prevent black spot algae?

  • Always disinfect any items used in the ocean prior to using them in your pool
  • Keep on top of your pool chemistry. You can get a complete computer water analysis at Aquafirst
  • Brush regularly, especially areas that may not have high levels of chlorine, such as steps, corners and behind ladders.
  • Keep your pool free from phosphates. Phosphates can increase the growth of algae