Black spot: treatment and prevention

What black spot is

Black spot is not a true algae. It is a colony of cyanobacteria (blue green bacteria) that forms hard, raised, dark blue green to black spots. Each spot builds a protective outer layer, so normal chlorine levels kill the surface but not the growth underneath.

Spores come into the pool on swimwear, toys, floats, cleaning equipment, wind and rain. Anything that has been in the sea, a lake, river or dam is a common source. Once in, spores survive in filter sand or cartridges, hoses, cleaners and baskets, which is why black spot often comes back after treatment. It grows best where water moves slowly and chlorine is low: steps, corners, behind ladders and shaded areas.

Reset everything before you swim

• Sand filter: replace the sand (or change to glass media).
• Cartridge filter: soak overnight in cartridge cleaner, hose clean, then soak in a chlorine solution. Replace it if it is old or damaged.
• Baskets, hoses, automatic cleaner, brushes and pole: clean and soak in a strong chlorine solution, or replace worn parts.
• Toys, floats, swimwear and wetsuits: wash and soak, or replace. Rinse anything used at the beach, a lake or a river.
• Water: have it tested and balanced at your pool shop.

Water targets

• Free chlorine: 2 to 4 ppm
• pH: 7.2 to 7.6
• Total alkalinity: 80 to 120 ppm
• Stabiliser: 30 to 50 ppm for an outdoor chlorinated pool
• Phosphates: keep low

Always follow product labels and your pool shop's advice for your pool.

Ongoing routine

• Weekly: brush the whole pool, especially steps, corners and shaded spots. Empty the baskets. Test chlorine and pH.
• After heavy use, rain or heat: shock dose and run the filter longer.
• Every day: run the filter long enough to turn the water over at least once.
• Monthly: full water test at your pool shop.
• Every 6 to 12 months: thoroughly clean the filter. Replace sand and cartridges when the manufacturer recommends.
• Algaecide: a non copper algaecide can help. Avoid copper based products, which can stain pool surfaces.

If you see a dark spot

Act early. Brush it firmly with a stiff nylon brush (never a wire brush, scourer or acid on your surface), shock the pool and clean the filter afterwards. If it keeps coming back, send us a photo and your latest water test and we can help you work out what it is.