A green pool is one of the most common service calls — and one where homeowners most often try to throw money at the problem without solving it. Two bags of shock, a bottle of algaecide, wait three days, still green. The reason is almost always a failure to execute the SLAM protocol correctly: either the wrong chlorine target, failure to maintain it, or stopping before the pool truly passes. This is the step-by-step guide for clearing any green pool the right way.
Not all green pools are the same. The severity determines how much chemical and time the job will require.
| Severity Level | Description | Expected Treatment Time |
|---|---|---|
| Level 1 — Light green | Slight green tint, bottom clearly visible | 24–48 hours |
| Level 2 — Green | Definite green, bottom visible in shallow end only | 2–4 days |
| Level 3 — Dark green | Opaque green, bottom not visible in any area | 4–7 days |
| Level 4 — Black/brown swamp | Opaque dark water, standing debris, possible dead animals | 7–14 days or drain |
Get baseline readings for at minimum: pH, CYA, and FC. Alkalinity and calcium hardness are secondary but worth noting. The CYA level determines your SLAM chlorine target — you cannot proceed without knowing it.
Before adding any chlorine, lower pH to 7.2 using muriatic acid. This ensures the maximum fraction of chlorine exists as active HOCl — the form that kills algae. Adding chlorine to a pool at pH 8.0 is wasteful; only ~22% of the chlorine is in the active form. At 7.2, approximately 66% is active. This step alone can shorten recovery time by 30–50%.
Use your CYA reading to determine the SLAM target. The target is approximately 40% of CYA (see the full table in the SLAM Protocol guide). Use liquid chlorine (sodium hypochlorite, 12.5%) — not cal-hypo, not dichlor. Adding CYA during treatment moves the goalposts; adding calcium to already-hard water creates scaling risk.
| Pool Volume | To raise FC by 10 ppm | To raise FC by 20 ppm | To raise FC by 30 ppm |
|---|---|---|---|
| 10,000 gal | 0.7 gal | 1.4 gal | 2.1 gal |
| 15,000 gal | 1.0 gal | 2.0 gal | 3.1 gal |
| 20,000 gal | 1.4 gal | 2.8 gal | 4.2 gal |
| 30,000 gal | 2.1 gal | 4.2 gal | 6.3 gal |
Note: These are the quantities needed to hit the target from zero. If the pool already has some FC (even in a green pool there may be residual), adjust accordingly.
Algae clings to pool surfaces in biofilm. Chlorine alone cannot penetrate this biofilm efficiently — physical disruption is required. Use a stiff pool brush on every surface: walls, floor, steps, corners, behind ladders, around light fixtures. This is not optional and is not a once-per-day job. Brush at least twice daily during SLAM, ideally three times.
For plaster and gunite, a nylon or stainless steel combination brush works well. For vinyl liners, use a softer nylon brush to avoid abrasion. For fiberglass, a soft pool brush only.
The filter is doing the heavy lifting — removing dead algae and suspended particles from the water. A filter loaded with dead algae provides a massive organic load that consumes chlorine rapidly. Plan to:
This is where most DIY green pool attempts fail. Adding a large initial dose and then not checking for 24 hours means FC dropped to near-zero within hours as the algae consumed it. The pool may look slightly better but the algae was not killed — just stunned. Test FC every 2–4 hours during active treatment and redose whenever it drops below the SLAM minimum.
Use an FAS-DPD test kit (Taylor K-2006) — standard DPD tests max out at 3–5 ppm and will read zero even at high chlorine levels, leading to massively incorrect dosing decisions.
SLAM is complete only when ALL THREE of these conditions are met on the same day:
After SLAM is complete:
PoolLens logs FC readings, OCLT results, filter cleaning notes, and treatment history — so you can document your green pool recovery for every account. Build a chemical record that proves the work was done. Free and offline.
Open PoolLens Free →The fastest method is: (1) Lower pH to 7.2 with muriatic acid, (2) Raise FC to SLAM level based on CYA, (3) Brush all surfaces twice daily, (4) Run filter 24/7 and clean every 8–12 hours, (5) Redose chlorine every 2–4 hours to maintain SLAM level. Light green water can clear in 24–48 hours. Heavier algae blooms take 3–7 days.
For a green pool SLAM at 30 ppm CYA, you need to raise and maintain FC at 12 ppm. For a 10,000-gallon pool with 0 ppm FC, raising to 12 ppm requires approximately 0.84 gallons of 12.5% liquid chlorine. But SLAM requires maintaining that level — expect to add 2–4 times that amount over the first 24 hours as chlorine is consumed by algae oxidation.
Treat the pool chemically unless: CYA exceeds 90–100 ppm, the pool has been green for months with severe surface staining, the filter is damaged, or it is faster and cheaper to drain. Most green pools clear with 3–7 days of sustained SLAM treatment.
Recurring green pools are almost always caused by: (1) CYA too high, making effective chlorination difficult, (2) Not completing the SLAM — stopping before passing the overnight chlorine loss test, (3) Poor circulation leaving dead spots where algae survives, (4) Filter not cleaned during treatment, allowing dead algae to redissolve.