Algae spores are in every pool. They're introduced by wind, rain, swimwear, and pool toys — and they're impossible to keep out entirely. What you can control is whether those spores get the conditions they need to grow.
Green algae is the most common type. It turns water green and makes surfaces slippery. It thrives when chlorine levels drop, circulation is poor, or pH drifts too high. A weekly shock and consistent chlorine levels (2–4 ppm) will keep it at bay.
Black algae is far more stubborn. It appears as dark spots on plaster or grout and embeds itself deep into the surface. It has a protective outer layer that standard chlorine struggles to penetrate. Treatment requires physical brushing with a steel brush followed by high-dose chlorine or specialized algaecide. It almost always comes back unless the source is addressed — usually contaminated swimwear or equipment from another pool.
Mustard algae looks like sand or pollen sitting on the pool floor and walls. It's chlorine-resistant and can survive on equipment, toys, and even swimwear outside the pool. Treatment requires a thorough shocking of both the pool and everything that touches it.
Prevention comes down to four things: consistent sanitizer levels, proper pH, good circulation, and regular brushing. Algae loves stagnant water and surfaces that aren't brushed regularly. Running your pump for at least 8 hours a day and brushing walls weekly removes the foothold algae needs to establish itself.
A phosphate remover added monthly can also help — phosphates are algae's primary food source, and keeping them low starves potential blooms before they start.
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