Green Pool Recovery - From Algae Bloom to Clean Water

Why this matters

A green pool - visible algae bloom, water from cloudy-green to opaque-black-green - is one of the highest-value pool service calls. Customers can't swim, can't see the bottom, sometimes can't even tell if the dog or a child fell in. The job has a clear scope, a measurable result, and a customer who's emotionally motivated to pay for fast resolution. A competent service routine recovers most residential green pools in 3-7 days; severe cases (deep black-green) can take 1-2 weeks. This article covers the procedure.

When to use this procedure

• Visible green tint in clear water (early algae bloom)
• Cloudy green water with visible particles
• Dark green opaque water (advanced bloom)
• Black-green pool floor invisible (severe bloom)
• After a power outage, equipment failure, or extended customer absence

Tools and materials

• Test kit (FAS-DPD chlorine test for high-range readings)
• Pool brush (stiff bristles for plaster; nylon for vinyl or fiberglass)
• Telescopic pole + leaf rake
• Vacuum head + hose
• Liquid chlorine (sodium hypochlorite, 10-12% concentration) - bulk gallons
• OR calcium hypochlorite shock (granular, 65-73%)
• Algaecide (polyquat 60% for general; copper-based for severe)
• pH adjusters (muriatic acid or sodium carbonate as needed)
• Cyanuric acid (CYA / stabilizer) test
• Polymer-based clarifier
• Filter cleaner (DE, cartridge, or sand-specific)
• PPE: chemical-resistant gloves, eye protection, respirator for shock handling

Safety

Customer pets must be kept away during shocking. Pool deck signage if the pool is in a multi-family or shared property.

Procedure

Step 1: Assess the bloom and the equipment

Walk the property and document:

1. Color depth: light green = early; opaque dark green = advanced; black-green = severe
2. Pool floor visibility - can you see the main drain?
3. Filter pressure on the gauge (high pressure = clogged filter; expected post-shock)
4. Pump and motor - operating normally?
5. Heater (if present) - should be OFF during recovery
6. Salt cell (if saltwater pool) - should be OFF; chlorine generation will be supplemented by liquid shock
7. Photo documentation of starting state

Step 2: Test the water

Critical baseline before adding chemistry:

1. pH (should be 7.2-7.4 for effective shock)
2. Total alkalinity (80-120 ppm typical)
3. Cyanuric acid (CYA / stabilizer) - this is the key reading
4. Free chlorine (typically 0 or very low in a green pool)
5. Combined chlorine
6. Total dissolved solids if available

The CYA reading determines the shock dose. The relationship between CYA and effective chlorine is the central concept:

Higher CYA means more chlorine is required to actually kill the algae. The Trouble Free Pool community's "FC/CYA chart" is the widely-accepted reference. If CYA is over 100 ppm, partial water replacement should be considered before shocking - the CYA can't be lowered chemically.

Step 3: Adjust pH if needed

1. If pH is above 7.5, add muriatic acid to bring it down to 7.2-7.4
2. Chlorine is far more effective at lower pH
3. Wait 30 minutes after acid addition; re-test

Step 4: Clean what's accessible

1. Skim surface debris
2. Brush every accessible surface - walls, steps, around skimmers, ladder
3. Vacuum the main floor if visibility allows
4. If visibility is too poor, skip to chemistry first; manual vacuuming can wait

Step 5: Backwash or clean the filter

The filter is going to be hit hard by algae particles. Pre-clean:

1. Sand filter: backwash thoroughly
2. DE filter: backwash, then recharge with fresh DE
3. Cartridge filter: remove, hose clean, soak in filter cleaner if recently neglected

Step 6: Shock the pool

Calculate the chlorine needed based on pool gallons and target FC:

1. Pool gallons × target FC PPM × 0.0007 = ounces of 12% sodium hypochlorite (For example, 20,000 gallons × 20 ppm × 0.0007 = 280 fl oz = 2.2 gallons)
2. Add the calculated chlorine with the pump running
3. For severe cases, an algaecide can be added at the same time (polyquat 60%)
4. Distribute around the pool perimeter for even mixing
5. Run the pump 24/7 until recovery is complete

Step 7: Monitor and re-dose

Throughout recovery (typically 3-7 days):

1. Test free chlorine every 4-6 hours during the day
2. Maintain shock-level chlorine continuously (re-dose every time FC drops below the shock target)
3. The pool will progressively clear:

• Day 1: very heavy chlorine demand; lots of dead algae particles
• Day 2-3: cloudy gray water (the dead algae)
• Day 4-5: cloudy white water (filter doing its work)
• Day 6-7: clearing to visible bottom

1. Brush daily to expose any algae hiding in corners

Step 8: Manage the filter

The filter takes a beating during green-pool recovery:

1. Watch the pressure gauge
2. Backwash sand and DE filters when pressure rises 8-10 PSI above clean baseline
3. Cartridge filters: pull, hose clean, reinstall (multiple cycles per day in heavy recovery)
4. Add a polymer clarifier to help the filter trap fine particles

Step 9: Recovery verification

The pool is recovered when:

1. Free chlorine holds at the shock target for 8+ hours overnight (algae is dead, chlorine demand is gone)
2. Water is clear; main drain visible
3. No green tint anywhere
4. Free chlorine drops to normal range (1-4 ppm typical for residential)

Step 10: Return to normal

1. Allow the chlorine to drop naturally to 3-5 ppm before allowing swim
2. Re-balance pH and alkalinity
3. Re-set CYA to optimal range (30-50 ppm typical residential)
4. Restart heater and salt cell
5. Resume normal weekly service cadence
6. Document for the customer file

Special situations

Mustard / yellow algae:

• Yellow-tinted slime, particularly in shaded areas
• More resistant to chlorine than green algae
• Often requires copper-based algaecide in addition to shock
• Brush vigorously; some forms shelter under the surface

Black algae:

References

• APSP / PHTA (Association of Pool & Spa Professionals) - water chemistry standards
• CDC Model Aquatic Health Code
• World Health Organization Guidelines for Safe Recreational Water Environments
• EPA registered algaecide product labels
• Trouble Free Pool FC/CYA Chart (widely accepted in the trade)
• Manuall internal: Algae Treatment SOP, Cloudy Water Diagnostic, Water Chemistry Balancing