Guides / Algae Treatment
How to Get Rid of Pool Algae — and Keep It Gone
Algae means chlorine lost the fight — usually low sanitizer, poor circulation, or both. Identify your algae type below, then treat aggressively in the correct order: brush, balance, shock in the evening, filter nonstop, vacuum, and retest. Half measures grow resistant algae; a complete sequence clears the pool in days.
Last reviewed: September 23, 2026 by the Pool Calculate Hub experts.
Identify your algae type first
Treatment intensity scales with type. Misidentifying black algae as green is the most common reason treatments stall halfway.
| Type | Appearance | Difficulty |
|---|---|---|
| Green (most common) | Water turns green and cloudy; walls feel slimy | Easiest — responds to standard shock plus brushing |
| Yellow / mustard | Dusty yellow patches in shady spots, on toys and equipment | Resistant — needs higher sustained chlorine and exhaustive brushing |
| Black | Raised dark spots with roots in plaster pores and cracks | Hardest — requires steel-brush spot treatment and multi-day elevated chlorine |
The kill sequence (in this exact order)
- Brush everything first. Walls, floor, steps, benches, ladders, skimmer throats, and behind light niches. Brushing breaks algae's protective biofilm — unbrushed algae survives shocking. For black spots on plaster, use a stainless-steel brush (never steel on vinyl or fiberglass).
- Balance pH to 7.2–7.4. Chlorine efficacy collapses as pH rises. Test with fresh reagents and correct pH before spending a dollar on shock. Confirm targets in our Chemistry Basics guide.
- Shock hard in the evening. Green water: reach breakpoint (roughly 10× combined chlorine). Yellow and black: higher sustained levels over multiple days. Evening dosing avoids sunlight burn-off. Size every dose to your exact volume with our Chlorine Calculator.
- Run the filter 24/7. Backwash sand/DE or rinse cartridges whenever pressure climbs 8–10 psi over clean. Empty baskets daily. Cloudiness that worsens after backwashing usually means the filter needs a rinse cycle first.
- Vacuum dead algae, brush again, retest. Vacuum slowly to waste where possible. Re-brush and re-shock any remaining green. You are done when the pool holds 1–3 ppm free chlorine overnight with no loss and water is clear. Do not swim until chlorine is back under 4 ppm.
Type-specific tactics
Green algae
Standard sequence works. Most green pools clear in 2–4 days of brushing, evening shock, and continuous filtration. The overnight chlorine-loss test is your finish line: dose in the evening, test at night and again at dawn — zero loss plus clear water means victory.
Mustard / yellow algae
Brush shady walls, pool toys, ladders, and automatic cleaner parts — mustard algae hides on equipment and reinfects the pool. Shock higher and longer than for green, keep the pump running around the clock, and wash anything that lived in the pool during the bloom.
Black algae
Spot-scrub every head with a steel brush to crack its waxy cap, then hold elevated chlorine for days — not hours. Expect several brush-and-shock cycles. Deep-rooted spots in cracked plaster may need professional resurfacing if they return season after season.
Prevention: the weekly routine that works
- Never let free chlorine sit at zero — hold 1–3 ppm consistently, not in spikes.
- Circulate a full turnover daily (typically 8–12 hours of pump runtime).
- Keep CYA at 30–50 ppm so sunlight does not strip chlorine by afternoon.
- Brush weekly even when water looks perfect — visible algae is the late stage.
- Shock weekly during heavy use, after storms, and after parties.
- Use algaecide as a preventative after water is clear — never as the cure.
Confirm your pool's gallons first with theVolume Calculator — every prevention dose depends on it.
What not to do
- Do not add algaecide to a green pool and stop there. Algaecide prevents; sustained chlorine plus brushing kills.
- Do not drain a vinyl-liner pool to fix algae. Liners shift, wrinkle, and tear. Treat chemically.
- Do not swim in an active bloom. Visibility and sanitation are both compromised — it is unsafe by definition.
- Do not shock at noon with high pH. Sunlight plus weak chlorine wastes the treatment. Evening, pH near 7.2.
Frequently asked questions
Will algaecide clear my green pool?
No. Brush and shock with chlorine first; add algaecide afterward to prevent regrowth once water is clear.
How long until I can swim?
When water is clear, free chlorine reads 1–3 ppm (below 4 ppm minimum), and pH reads 7.2–7.6. Usually 24–48 hours after the final shock for green algae; longer for black.
Why does algae keep returning?
Chlorine hitting zero between doses, wrong CYA, too little pump runtime, or skipped brushing. Fix all four — spot-fixing one rarely holds.
Keep going
- Chlorine Calculator — exact shock and daily doses for your volume.
- Chemical Calculator — pH and alkalinity correction before shocking.
- Troubleshooting Cloudy Water — when haze remains after algae is dead.
- Pool Chemistry Basics — the ranges that prevent the next bloom.
Print this guide for treatment day.