Black Spots and Slippery Film in a Pool, Algae, Mould or Staining?
Black, green or slippery patches are often called mould, but the cause may be algae, biofilm, a metal stain or deterioration in the pool surface. Correct identification matters because each problem needs a different response.
What does the mark look and feel like?
| What you see | Possible cause |
|---|---|
| Green dust that brushes away | Active algae or debris settling in a low flow area |
| Dark spots with a stubborn centre | Black algae, staining or a surface defect |
| Slippery clear or pale film | Biofilm caused by poor sanitation or circulation |
| Brown, rust or purple marks | Metal or organic staining |
Check the water and circulation
Growth often begins where circulation is weakest, including steps, corners, behind ladders and around lights. Check sanitiser, pH and the wider water balance. Empty baskets, review filter pressure and confirm that return jets are moving water across the affected area.
Brush a test area
Use a brush suited to the pool surface. A mark that releases as a cloud is different from one embedded in the finish. Do not use a steel brush on fibreglass, vinyl or a surface that can be scratched.
When the surface is the real problem
Old plaster, beadcrete and damaged coatings can become rough or porous. Growth and dirt return because they lodge in the surface, even after the water is treated. Repeated aggressive acid washing can make an already tired surface worse.
A sensible treatment sequence
- Remove loose debris and clean the baskets.
- Test and correct the water using products suitable for the pool.
- Brush the affected areas with the correct brush.
- Run filtration and clean the filter as the material is captured.
- Reassess the remaining marks before using a stain treatment.
Do not combine algae treatment, acid and stain products unless the product directions specifically permit it. If the mark keeps returning in exactly the same places, investigate circulation and surface condition rather than repeating the same chemical dose.
For a badly affected pool, see our emergency pool help and drain and clean services.
Not sure what you are looking at?
Send Pool Perfectionists a photo and a short description, or book a Sydney pool technician to inspect the problem properly.