Real-life case: the embedded black spots that resisted everything
- The situation: small blue-black spots embedded in the surface, coming back despite chlorine and light brushing.
- Black algae form a protective layer: normal chlorine slides over them without reaching them.
- The key: break the film with VIGOROUS brushing (stiff brush) before the boosted chlorination — otherwise the treatment never gets in.
- Full method: brush hard, treat both locally and across the whole pool, persevere.
At first, you think it’s just dirt. Small blue-black dots, on the surface, near the shaded end of the deep section. A light pass with the brush: they stay. A bit of extra chlorine: they stay. A few days later: there are more of them. Black algae are the pool’s champions of resistance — and the reason so many owners give up. Here’s how this owner finally won.
Why they resist everything
Black algae have nothing to do with green water. Green water floats; black algae embed themselves. They anchor into the pores of the surface, the grout lines, the micro-cracks, and above all they coat themselves in a watertight protective film. The result: chlorine, even at the right level, slides over them without reaching them. That’s the whole misunderstanding — you dose, you wait, nothing moves, and you conclude that “chlorine doesn’t work”. In reality, it simply isn’t touching the algae.
The practical consequence is counter-intuitive: against black algae, the decisive move isn’t chemical, it’s mechanical.
The method that worked
1. Brush hard — really hard. Stiff brush (suited to the surface), over every spot, until the film breaks and the algae opens up. This is the step nobody does enough of: without it, everything else is pointless. On stubborn areas, some scrub spot by spot.
2. Treat, across the pool and locally. A boosted chlorination for the whole body of water, calculated on the volume — the app handles the dose and the order (pH corrected first, always). Plus special attention to the affected areas, right after brushing them, while they’re “open”.
3. Continuous filtration and patience. Black algae don’t surrender in an hour. The brushing is repeated every day for several days, the treatment is maintained, and you don’t declare victory at the first sign of clearing: a single root forgotten in a grout line restarts everything.
4. Re-test and monitoring. Once the spots are gone, the areas stay under observation: recurrence always comes from a survivor in a micro-crack.
What this case teaches
Black algae are the case where elbow grease counts as much as chemistry: no vigorous brushing, no victory, whatever the dose. It’s also a case of persistence — they yield to repeated method, not to a single dramatic effort. The app frames the chemical side (pH first, boosted chlorination dosed for your pool, scheduled re-tests) and reminds you of the daily brushing discipline. The rest is consistency — and it pays off.
Frequently asked questions
How do I recognise black algae?
Dark spots (blue-black, green-black), small and well defined, firmly anchored to the surface, especially in shaded areas, grout lines and micro-cracks. Unlike suspended green algae, they're embedded and won't lift with a simple pass of the net.
Why doesn't chlorine kill my black algae?
They protect themselves under a watertight film and anchor into the pores of the surface. Chlorine slides over the top without penetrating. You first have to break that layer with vigorous brushing so the treatment can reach the algae itself.
Do black algae always come back?
They can recur if a single root survives in a grout line or a crack. That's why it matters to brush hard, treat with extra focus on the affected areas, and persevere over several days rather than stopping at the first sign of improvement.