Those small black or dark blue-green spots on your pool walls and floor are almost always black algae. It is the most stubborn type of pool algae, and the usual shock-and-hope approach will not shift it.
Why black algae is so hard to kill
Black algae is different from the green algae that turns a pool cloudy. It grows in colonies that anchor roots into tiny pores in plaster, pebblecrete and tile grout. Each colony also grows a waxy protective layer that shields it from chlorine.
That is why shocking the pool alone rarely works. The chlorine kills the surface layer and leaves the roots behind, and the spots return within weeks.
How to treat it
- Brush hard. Scrub every spot with a stiff brush to break the waxy layer. Use a stainless steel brush on concrete or pebblecrete. Stick to a firm nylon brush on fibreglass or vinyl, as steel will damage those surfaces.
- Test and balance the water. Chlorine works poorly when pH is high. Get pH into the 7.2 to 7.6 range first.
- Shock the pool. Raise chlorine well above normal levels so it can attack the exposed colonies.
- Add a black spot algaecide. Purpose-made black spot treatments penetrate the colonies far better than chlorine alone. We stock them instore and can advise on dose for your pool volume.
- Brush again, daily. Repeat brushing for several days so the treatment keeps reaching the roots.
- Run the filter long hours during treatment, and brush any returning spots immediately.
Why it comes back
If even a few roots survive, the colony regrows. Persistence beats black algae. One thorough treatment followed by quick attention to any returning spots works far better than repeated half-treatments.
How to stop it taking hold
- Brush walls, steps and shaded corners weekly, even when the pool looks clean
- Keep chlorine steady, not spiking and crashing between doses
- Test after heavy rain, which feeds algae by washing in phosphates and diluting sanitiser
- Wash swimmers and gear that have been in natural waterways, a common way black algae arrives