How to Stop Algae Growing in Your Pool (Complete Guide)

Stop Algae Immediately: The SLAM Method

Yesterday your pool looked like a green swamp that would make frogs jealous — by tonight it'll be crystal clear water you'd be proud to dive into. This means raising your free chlorine (FC) to shock level based on your cyanuric acid (CYA) level and keeping it there continuously until the algae is completely eliminated. Don't waste time with algaecides or "shock" products from pool stores - liquid chlorine is your most effective weapon.

Step 1: Test Your Water Chemistry

Before adding any chemicals, you need accurate readings of your current water chemistry. Use a quality test kit like the Taylor K-2006C to test:

• Free Chlorine (FC)
• Combined Chlorine (CC)
• pH level
• Total Alkalinity (TA)
• Cyanuric Acid (CYA)

Critical: If your CYA is above 80ppm, you'll need to partially drain and refill your pool first, as chlorine becomes ineffective at high stabilizer levels.

Step 2: Calculate Your Shock Level

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

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

For every 10ppm increase in CYA, add 4ppm to your shock level. This is why maintaining proper CYA levels (30-50ppm for regular pools, 70-80ppm for salt water generators) is crucial for algae prevention.

Step 3: Add Liquid Chlorine

Purchase liquid chlorine (sodium hypochlorite) from pool stores or grocery stores. Avoid calcium hypochlorite "shock" as it raises calcium hardness and can cause scaling.

To raise FC by 10ppm in a 20,000-gallon pool, add approximately 2.6 gallons of 12.5% liquid chlorine. For a 10,000-gallon pool, use 1.3 gallons. Add the chlorine in the evening to prevent sun degradation, pouring it around the pool perimeter with the pump running.

Step 4: Maintain Shock Level 24/7

This is where most pool owners fail. You must maintain shock level around the clock until you pass the Overnight Chlorine Loss Test (OCLT). Test FC twice daily - morning and evening - and add more liquid chlorine to maintain your target level.

Algae consumes chlorine rapidly, so expect to add significant amounts initially. As the algae dies off, your chlorine consumption will decrease. Don't get discouraged if you're adding chlorine multiple times per day at first.

Step 5: Brush Aggressively

Brush your pool walls, steps, and floor thoroughly twice daily. Algae forms protective biofilms that chlorine can't penetrate effectively. Brushing breaks up these colonies and exposes algae to the sanitizer. Pay special attention to areas with poor circulation like steps, behind ladders, and corners.

Use a stainless steel brush for concrete pools or a nylon brush for vinyl and fiberglass surfaces. Brush from shallow to deep end, and always brush before adding chemicals.

Step 6: Run Filtration Continuously

Keep your pump running 24/7 during the SLAM process. Proper circulation is essential for distributing chlorine and capturing dead algae. Clean or backwash your filter when pressure rises 8-10 PSI above the clean starting pressure.

If you have a cartridge filter, you may need to clean it daily during heavy algae blooms. For sand filters, backwash when needed but don't over-backwash as this wastes water and chemicals.

Monitor pH and Alkalinity

Liquid chlorine has a high pH (around 13), so it will raise your pool's pH over time. Keep pH between 7.2-7.6 for optimal chlorine effectiveness. Add muriatic acid to lower pH when it exceeds 7.8.

Maintain total alkalinity between 80-120ppm for regular pools or 60-80ppm for salt water generator pools. Proper TA helps stabilize pH and prevents rapid pH swings.

The Overnight Chlorine Loss Test

You'll know the algae is gone when you pass the OCLT. After maintaining shock level for at least 24 hours with clear water, test FC in the evening and again the next morning. If FC drops less than 1ppm overnight, the algae is dead and you can return to normal chlorine levels.

Prevention Strategy

Maintain FC levels based on your CYA - typically 4-6ppm FC with 30-50ppm CYA. Test chlorine 2-3 times weekly and brush weekly. Keep pH between 7.4-7.6 and ensure proper circulation. Address algae immediately when you first notice it, as early intervention prevents major blooms.

Warning: Never swim in a pool during SLAM treatment. The high chlorine levels can cause skin and eye irritation.

Tools & Supplies You'll Need

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