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Excessive Algae/Biofilm — Insufficient Algicide or Sunlight Exposure

Likely causes

Insufficient sunlight blocking on basin/fill — creates photosynthesis environment for algae

Check: Check for green/brown discoloration on basin and fill; measure residual chlorine (below 0.1 ppm = insufficient biocidal activity)

What to do: Physically remove algae from basin and fill with brush scrubbing and high-pressure wash → shock algicide treatment (DBNPA or quaternary ammonium at 2–3× concentration) → install UV-blocking covers or opaque panels

Algicide not dosed or overdiluted interval — residual chlorine below 0.1 ppm

Check: Review algicide dosing records; if residual chlorine below 0.3 ppm, re-evaluate dosing interval and concentration

What to do: Maintain residual chlorine 0.3–0.5 ppm + establish weekly regular algicide dosing → clean leaves and debris around basin weekly → monthly ATP test to monitor biofilm load

Field tip

When algae appear, also check for odor — a rotten egg smell (H₂S) signals sulfate-reducing bacteria and requires immediate action.

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