Diagnose the green first
Light green = early algae, recoverable in 3 days. Dark green / pea-soup = established bloom, 5–7 days. Black spots on marbelite = black algae, specialist treatment. Brown/yellow tinge = metals (often iron from borehole top-up), needs sequestrant not chlorine.
The recovery sequence
Do this in order — skipping a step costs you days:
- Test pH and adjust to 7.2 (low pH makes chlorine work harder)
- Backwash or clean the filter completely
- Shock dose: 300–500 g calcium hypochlorite per 10,000 L (or equivalent stabilised chlorine)
- Brush all walls and floor, especially shaded corners
- Run pump 48 h continuously
- Add flocculant per dosage, let dead algae settle 12–24 h
- Vacuum settled debris to waste (not back through the filter)
- Test, balance, top up water lost during vacuum-to-waste
Cost of recovery (2025 ZAR)
A 35,000-litre family pool:
- Mild green: R600–R1,200 in chlorine, pH adjuster, flocculant
- Dark green: R1,400–R2,400
- With water replacement (severe black algae): R2,400–R3,200 plus R350–R800 in municipal water
- Service-plan callout: R650–R1,200 visit + chemicals at retail markup
What prevents the next bloom
Maintain free chlorine 1.5–3 ppm with cyanuric acid (stabiliser) 30–50 ppm. Don't let pH drift above 7.6. Run filtration 8 h/day in summer. Brush walls weekly — algae spores anchor in the marbelite micropores before they're visible.