How to Clean a Green Pool: Complete Step-by-Step Guide

Understanding Why Your Pool Turned Green

Green pool water indicates algae growth, which occurs when chlorine levels drop too low to sanitize effectively. Common causes include equipment failure, inadequate chlorination, poor circulation, or chemical imbalances. The green color comes from chlorophyll in algae cells, and the intensity indicates the severity of the problem.

Essential Testing and Water Balance

Before adding any chemicals, test your water using a reliable test kit like the Taylor K-2006C. You need accurate readings for:

• Free Chlorine (FC): Currently near zero in green pools
• Combined Chlorine (CC): Should be less than 0.5 ppm
• pH: Target 7.2-7.6 for effective chlorination
• Total Alkalinity (TA): 80-120 ppm for regular pools, 60-80 for salt water
• Cyanuric Acid (CYA): Critical for determining shock level
• Calcium Hardness (CH): 250-350 ppm for plaster pools

If pH is above 7.6, lower it using muriatic acid before proceeding. High pH reduces chlorine effectiveness significantly.

The SLAM Method: Your Green Pool Solution

Step 1: Calculate Your Shock Level

Your shock level depends entirely on your CYA level. Use this FC/CYA relationship:

• CYA 30 ppm = FC shock level 12 ppm
• CYA 40 ppm = FC shock level 16 ppm
• CYA 50 ppm = FC shock level 20 ppm
• CYA 60 ppm = FC shock level 24 ppm
• CYA 70 ppm = FC shock level 28 ppm

Warning: If your CYA exceeds 80 ppm, you'll need to partially drain and refill your pool before proceeding, as shock levels become impractically high.

Step 2: Add Liquid Chlorine

Use liquid chlorine (sodium hypochlorite), not granular "pool shock" from stores. Calculate dosing: 1 gallon of 10% liquid chlorine raises FC by approximately 1.3 ppm in 10,000 gallons of water. Add chlorine in the evening to minimize sun degradation, pouring around the pool perimeter with circulation running.

Step 3: Maintain Shock Level

Test FC levels twice daily (morning and evening) and add chlorine as needed to maintain shock level. Initially, chlorine will drop rapidly as it kills algae. This process typically takes 3-7 days depending on algae severity. The pool will gradually change from green to cloudy white/blue, then clear.

Critical Daily Maintenance During SLAM

Brush your pool walls, steps, and floor thoroughly twice daily using a stainless steel brush for concrete pools (if needed for stubborn algae) or nylon brush for vinyl/fiberglass and most concrete pools. Brushing dislodges algae from surfaces, allowing chlorine to kill it more effectively. Run your filtration system 24/7 during the SLAM process.

Clean your filter every 2-3 days during SLAM. For sand filters, backwash when pressure rises 8-10 psi above clean pressure. For cartridge filters, rinse with a hose or replace if severely clogged. DE filters should be backwashed and recharged with fresh DE.

Completing the SLAM Process

Your SLAM is complete when you achieve all three criteria:

1. Water is crystal clear
2. FC doesn't drop more than 1 ppm overnight
3. Combined Chlorine (CC) is 0.2 ppm or less

Once complete, vacuum the pool thoroughly to remove dead algae debris. You may need to vacuum to waste if debris is heavy, then refill and rebalance as needed.

Common Mistakes to Avoid

Never use algaecide during SLAM - it interferes with chlorine and prolongs the process. Avoid using trichlor tablets or granular shock, as these contain CYA and will raise your stabilizer levels. Don't rush the process by adding extra chlorine beyond shock level - this wastes money without improving results.

Maintain proper pH throughout SLAM. If pH drifts above 7.6, add muriatic acid immediately. High pH can stall the entire process.

Prevention for the Future

Maintain proper FC levels based on your CYA (typically 3-5 ppm for 30-50 CYA). Test water 2-3 times weekly during swimming season. Ensure adequate circulation - run pumps long enough to turn over your entire pool volume daily. Keep pH between 7.4-7.6 and TA properly balanced.

Regular brushing prevents algae attachment, especially in areas with poor circulation. Address equipment problems immediately, and never let chlorine levels drop to zero.

Tools & Supplies You'll Need

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