Green pool at opening is the most common spring service call. The cause is almost always the same: algae spores that survived the winter in an undertreated or early-closed pool bloomed as temperatures warmed under the cover. Recovery is a systematic process — not a matter of dumping in shock and hoping. The sequence, the pH management, and the filter management all determine how fast the water clears.
Green pools are not all the same problem. The severity determines the treatment dose and realistic recovery timeline:
| Severity | Appearance | Shock Dose | Recovery Time |
|---|---|---|---|
| Light green | Tinted water, bottom visible | 2× standard dose | 2–3 days |
| Medium green | Cloudy green, bottom partially visible | 3× standard dose | 3–5 days |
| Dark green | Opaque green, bottom not visible | 4× standard dose | 5–7 days |
| Black/swamp | Dark, thick, visible debris | 5× or more; multiple treatments | 7–14 days |
Standard dose reference: 1 lb of calcium hypochlorite (65%) per 10,000 gallons for normal maintenance shock. For green pool recovery, multiply by the severity factor above.
Before adding any chemicals, confirm the pump, filter, and plumbing are operational. No chemistry works without circulation. Check:
If filter pressure is already at or near the backwash threshold, do one backwash before beginning green pool treatment. Starting treatment with a heavily loaded filter will cause pressure to spike rapidly and reduce treatment effectiveness.
This is the step most people skip, and it explains why shocking alone often fails. Free chlorine activity is dramatically reduced at high pH:
After a winter under a cover, pool pH is often 7.8–8.2 from alkaline debris and rain exposure. Shocking a pool at pH 8.0 wastes 80% of the shock's potential because most of the chlorine is in the inactive OCl⁻ form. Bring pH down to 7.2 — slightly below the normal target — before shocking. This maximizes the kill rate during the initial treatment.
Add muriatic acid (hydrochloric acid) per the label to lower pH. Broadcast into the deep end with the pump running. Wait 30 minutes and retest before shocking.
Pre-dissolve calcium hypochlorite in a bucket of water before adding to the pool — undissolved granules sinking to the bottom can bleach vinyl liners. For pools with plaster or concrete surfaces, you can broadcast directly with the pump running.
Add shock at dusk or after dark. Sunlight (UV radiation) destroys calcium hypochlorite rapidly when exposed directly — you can lose 50% of its potency in the first hour of sunlight. Adding at night gives the chemical the full dark period to circulate and kill algae before UV exposure begins.
For medium or dark green pools, add the full dose in a single evening. Run the pump continuously overnight at high speed — do not timer-restrict the pump during active algae treatment.
Brushing during treatment serves a critical function: algae attaches to surfaces and creates a protective barrier against the chlorine in the water. Brushing breaks this attachment and forces the algae cells into suspension where the chlorine can reach them directly.
Brush every surface: walls, floor, steps, behind ladders, corners. For medium and dark green pools, brush twice daily — morning and evening — during the recovery period. Use a stainless steel brush on plaster and concrete; nylon on vinyl and fiberglass.
As algae dies, it turns from green to grey-white and becomes suspended particulate in the water — this is good, it means the treatment is working. But this dead algae must be physically removed by the filter. Without active filter management, the dead algae recirculates and keeps the water cloudy.
After the first 24–48 hours of treatment, the pool will typically turn from green to milky-white or grey. This is normal — it means the algae is dead and now particulate. Continue circulating and backwashing; the clarifier helps coagulate this particulate for filter capture. Cloudy white water after a successful shock treatment clears within 24–48 hours of continuous filtration.
After the initial shock treatment (12–24 hours after adding shock), add a liquid clarifier (Bioguard Power Floc, Natural Chemistry Pool First Aid, HTH Super Clarifier). Clarifiers cause the fine dead algae particles to coagulate into larger clumps that the filter can capture more efficiently. Follow label dosing.
Do not add clarifier at the same time as shock — the high chlorine level will break down some clarifier formulations before they can work. Add clarifier after FC has started to drop from its initial shock peak.
For dark green pools with significant algae sediment on the bottom, vacuum to waste (not through the filter) to remove the bulk of the dead algae. Set the multiport valve to Waste, attach the vacuum head and hose, and slowly vacuum the pool floor while the water drains directly to waste.
You will lose water — plan to refill after vacuuming. The tradeoff is worth it: running dead algae through a filter loads it rapidly and forces constant backwashing, while vacuuming to waste removes the material completely without filter load.
If FC is zero again after 24 hours and the water is still green or cloudy, repeat the shock dose. Dark green pools often require 2–3 shock treatments on successive evenings before the algae load drops to a manageable level. Log every treatment, dose, and FC reading in SplashLens to track the recovery trend.
Log each shock treatment, FC reading, and filter backwash during green pool recovery. See the trend, document the timeline, and have full records if the customer asks why it took multiple visits to clear.
Open SplashLens Free →Light green pools typically clear in 2–4 days with aggressive treatment and continuous filtration. Dark green or black pools can take 5–7 days of treatment, multiple shocks, and extensive backwashing. Very heavy algae infestations can take up to 2 weeks.
No. Any visible green color indicates active algae growth, which means free chlorine is zero or critically low. Swimming in algae-contaminated water exposes swimmers to pathogenic bacteria. Do not allow pool use until water is clear and chemistry is confirmed normal.
Only in extreme cases — very dark or black water where algae has been established for months. Most green pools can be recovered with aggressive treatment without draining. Draining and refilling has its own risks and costs and should be a last resort.
Common reasons: pH too high (shock is ineffective above pH 7.8 — lower pH to 7.2 first), not enough shock for the algae load, shock added in daylight and destroyed by UV, or the filter is too dirty to remove dead algae from the water.