Black Algae in the Pool: Treatment That Works in Mallorca
Black spots that come back after every brushing? Black algae has roots in the grout. Here is how it is actually beaten, and when to call for help.
Why black algae is different
Black algae is not dirt on the surface. It anchors roots into grout lines and rough plaster, and it grows a protective layer that shrugs off normal chlorine levels. That is why it returns two weeks after every cleaning that only brushes the visible spot.
The treatment that works
The sequence matters: scrub each spot hard with a stainless steel brush to break the protective layer, shock the pool and hold chlorine high for several days, brush again daily while filtration runs long hours, and finish with a quality algaecide. Skipping the brushing is the classic failure, unbroken colonies simply regrow.
When to call a professional
If colonies cover large areas, sit deep in old grout or return a third time, the honest fix is professional treatment, sometimes combined with regrouting where the joints themselves have become the habitat. A monthly service contract at 150 to 250 EUR keeps chemistry stable so it never starts.
Prevention on this island
Mallorcan heat pushes chlorine consumption up fast in summer. Stable balance, weekly brushing of shaded corners and steps, and a clean filter are what keep black algae from ever finding a foothold.
Frequently asked questions about black algae
Why does black algae keep coming back?
Because the roots sit in the grout and the colonies wear a protective layer. Without hard brushing to break that layer before shocking, the visible spot dies and the root regrows.
Is black algae dangerous to swimmers?
The algae itself is not toxic, but a pool that supports it has weak sanitation, which is the real health concern. Fix the chemistry, not just the spots.
How long does treatment take?
A proper cycle takes about a week: breaking the layer, several days of high chlorine with daily brushing, then an algaecide finish.
Does salt water prevent black algae?
A well run salt system keeps chlorine stable, which helps a lot, but neglected balance lets algae in regardless of how the chlorine is produced.
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