How to Treat and Prevent Black Algae

If you’ve noticed stubborn, raised black or dark-green spots anchoring themselves to your pool walls and floor, you aren’t dealing with typical pool slime. You’re facing black algae. Unlike standard green variants, black algae behaves like an invasive weed. It grows protective, woven layers and deep physical “roots” that burrow down into porous surfaces like plaster, marbelite, and tile grout. This makes it incredibly resilient against standard chemical treatments — but with a solution-first strategy, it can be entirely eradicated.

How to Treat It

To treat black algae, you must aggressively break past its protective outer coating and destroy it at the root.

1. Balance the Water and Brush Vigorously

Unbalanced water paralyses your sanitisers. First, test your water parameters and adjust your pH to sit between 7.2 and 7.6. Once balanced, use a stiff pool brush to aggressively scrub the black spots. You must manually crack open the algae’s hard outer shell so the chemicals can penetrate the core.

2. Apply a Targeted Shock Treatment

With the outer shell cracked, introduce an aggressive oxidiser to destroy the organic spores. Add 100g–200g PoolBrite Month Mate Granular Xtra Blue per 10,000L (or 1–2 bags of Month Mate Shock Xtra Blue per 50,000L). For heavy infestations, always lean toward the higher dosage.

3. Deploy the Algaecide and Clarify

Follow up immediately by adding 1L PoolBrite Month Mate All-In-One Algaecide per 50,000L directly to the pool water. Keep your pump and filtration system running continuously to circulate the treatment. To help your filter pull the dead, microscopic algae spores out of suspension, add 2 x Brite-Blu Clarifier Tablets after your next backwash.

4. Decontaminate the Filter Sand

Standard backwashing cannot detach sticky black algae spores from your filter bed. Left untreated, the filter will simply reinfect your pool. Pour 1L PoolBrite Filter Cleaner into your three-bag sand filter to strip away organic biofilms, scale, and oils, ensuring the spores are chemically dissolved and flushed away.

How to Prevent It

Once your pool is sparkling clear, prevent a secondary outbreak by establishing a proactive maintenance routine:

  • Test Weekly: Keep your pH locked between 7.2 and 7.6 so your sanitiser works at peak efficiency.
  • Constant Sanitisation: Use a Month Mate Super Plus Floater to maintain a steady, unbreakable free chlorine residual of 1–3 ppm.
  • Monthly Protection: Dose your pool with 2L of Tru Blu Algaecide per 50,000L once a month as a preventative shield.
  • Routine Shocks: Oxidise hidden contaminants weekly using one bag of Month Mate Shock Xtra Blue per 50,000L.
  • Circulate and Scrub: Run your pump for 8–10 hours daily in summer (6–8 hours in winter), and brush the pool walls weekly to stop spores from gaining a physical foothold.