Black Algae in Your Pool: Why It Keeps Coming Back and How to Remove It
Black spots on the plaster or grout that come back no matter how much chlorine you throw at them? That is black algae — and it is a different enemy from the green bloom, because it roots into the surface itself.
Why shocking does not kill it
Black algae grows protective layers and anchors its roots into porous plaster and grout lines. Chlorine kills the surface layer; the roots survive and regrow within weeks. This is why owners fight the same spots for months.
How professional removal works
- Aggressive brushing with the right brush to break the protective layer
- Targeted chemical treatment directly on colonies, plus a strong sustained shock
- Repeat treatment cycles — single treatments almost never finish the job
- In entrenched cases, the honest fix is surface renovation, because the algae lives in the plaster itself
The bigger signal
Black algae usually appears in pools with aging, porous surfaces and patchy care. If it is widespread, treat it as a sign the surface is near end-of-life — see our renovation guide — and tighten the regular care that stops it re-establishing.
Persistent spots that survive shocking need proper treatment, not more chlorine.
Get black algae treated properly
Send a photo of the spots and we will confirm what it is and quote the treatment.