In this article, we will show you how to spot black spot algae in your pool, discuss the best way to kill black spot algae and what type of preventative measures you should put into place.
Black spot is a common algae problem found in fibreglass, pebble-crete, tiled and also marble-sheen pools. The problem with this algae strain specifically is its stubborn resistance to chlorine treatment. Its outer layers help to shield the algae from regular chlorine treatment, while its spores penetrate into pool surfaces.
Signs of Black Spot Algae
Knowing how to differentiate black spot algae from ordinary pool marks is the first step to applying the correct treatment before it takes over your pool.
Black, green or blue lumps
Appears as raised, discoloured spots rather than a flat stain or smear
Hides in shaded areas
Found predominantly in corners and shaded spots where sunlight doesn't reach
Slimy to the touch
Feels similar to moss this is the protective outer membrane that shields it from chlorine
Spreads and thickens
Left untreated, spots will grow larger and multiply across pool surfaces
Can You Swim in a Pool with Black Spot Algae?
The algae itself does not pose a direct harm to your health, however it will deplete your pool chlorine rapidly, making it a perfect breeding ground for harmful bacteria such as E. coli to thrive. Whether it's safe to swim depends on your current water quality and how advanced the spread is. When in doubt, treat first.
How to Get Rid of Black Spot Algae
Treatment combines a designated algaecide with high-strength granular chlorine. Together, they work to starve and clear the algae. Because black spot is protected by a tough outer membrane, physical scrubbing is just as important as the chemicals.
Brush vigorously with a stainless steel brush
This breaks down the algae's outer membrane so chemicals can penetrate. Don't use a standard nylon brush it won't be effective enough.
Turn pump off for 2 hours
Once the water has stilled, assess the algae and determine treatment severity before proceeding.
Apply chlorine based on severity
Leave pump off for 24 hours, then vacuum to waste
This step is critical skipping it is the most common reason black spot returns.
Add algaecide after 3 days
Allow 3 days after the chlorine treatment before adding algaecide for maximum effectiveness.
Rebalance your water after 36 hours
Take a water sample to your local pool shop and balance as directed by the technician.
Brush twice a week for 4 weeks
Continue brushing the affected area regularly to prevent regrowth while the algae fully clears.
Important Notes
Black spot will only reappear if all steps including step 4 have not been carried out in full.
Maintain correct water balance year-round for best algae defence: pH 7.2-7.6, Chlorine 2-3ppm.
How to Prevent Black Spot Algae
Save yourself time and money by preventing an outbreak before it starts. Consistent maintenance is far easier than treating an established colony.
Keep your equipment in good shape
Ensure good flow, filtration and sanitation a well-maintained system is your first line of defence.
Test pH, alkalinity and chlorine regularly
Don't wait for a visible problem regular testing catches imbalances before algae gets a foothold.
Brush and vacuum regularly
Pay particular attention to corners and shaded areas where black spot prefers to establish itself.
Run your filter 8-12 hours a day during swimming season
Consistent circulation keeps water sanitised and makes it harder for algae to establish.
