Black Spot in Pools: What It Is and How to Treat It
If you’ve ever noticed stubborn dark patches clinging to your pool walls or floor, you might be dealing with black spot algae — one of the most persistent and frustrating pool problems.
What is Black Spot Algae?
Black spot algae is a form of blue-green algae (cyanobacteria) that attaches firmly to rough pool surfaces like concrete, pebblecrete, or older plaster. It develops a protective layer that makes it resistant to normal chlorine levels, and it can burrow into tiny surface pits, making it incredibly hard to remove.
It usually starts as small black or dark blue dots, often in shady spots of the pool, and can slowly spread if not treated quickly.
Why is Black Spot a Problem?
Difficult to Kill: Its protective coating resists chlorine and most standard chemicals.
Surface Damage: It penetrates into porous surfaces, causing permanent marks.
Health Risk: While it’s not as immediately harmful as some bacteria, it can indicate poor water balance and create an environment where harmful organisms thrive.
How to Treat Black Spot Algae
Brush Thoroughly
Use a stainless steel algae brush (for concrete/pebblecrete pools) to break the algae’s protective layer. This step is crucial — without it, chemicals won’t penetrate.Shock the Pool
Raise your chlorine levels significantly with a strong shock treatment. For black spot, you’ll need very high chlorine levels sustained over several days. Always follow manufacturer instructions for dosage.Use a Black Spot Algaecide
Choose a specialised algaecide formulated for black spot. These products often contain copper or other potent algaecides that work alongside chlorine to kill algae deep in the surface.Maintain High Chlorine Levels
Keep your chlorine elevated for at least 3–5 days after treatment. Black spot won’t die overnight — persistence is key.Balance Your Water
After treatment, ensure your pH is in the ideal range (7.4–7.6), as this maximises chlorine effectiveness. Check alkalinity and calcium hardness to avoid future problems.Regular Brushing & Filtration
Even after it’s gone, keep brushing problem areas weekly and maintain strong circulation. This helps prevent re-growth.
How to Prevent Black Spot in the Future
Keep chlorine and pH consistently in range.
Brush walls and floors regularly, even if the pool looks clean.
Maintain good circulation and filtration, especially in corners and shaded spots.
Avoid letting leaves and debris settle for long periods.
Black spot algae is stubborn, but with the right approach — brush, shock, treat, and repeat — you can banish it and keep your pool looking fresh and inviting.