Kill pool algae using the SLAM method: shock your pool to the proper chlorine level based on your stabilizer, maintain that level until algae dies, and brush/vacuum debris.
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Pool algae appears when chlorine levels drop too low, allowing these microscopic plants to bloom and take over your water. The most effective way to kill algae is using the SLAM method (Shock Level And Maintain), which involves raising free chlorine to a specific shock level and maintaining it until all algae dies.
Unlike traditional "shock and wait" approaches, SLAM requires maintaining elevated chlorine levels continuously until your pool passes an overnight chlorine loss test, ensuring complete algae elimination.
Before adding any chemicals, test your water using a reliable test kit like the Taylor K-2006C. You need accurate readings for:
Critical: If your combined chlorine exceeds 0.5 ppm, you'll need to address chloramines before starting SLAM. High CC levels indicate your chlorine is tied up and won't effectively kill algae.
Your shock level depends on your CYA (stabilizer) level. Higher CYA requires more chlorine to be effective:
If your CYA exceeds 80 ppm, you'll need to partially drain and refill your pool before SLAM will be effective. High stabilizer levels make chlorine nearly useless against algae.
Add liquid chlorine (sodium hypochlorite) to reach your target shock level. Avoid calcium hypochlorite "shock" products during SLAM as they add calcium and raise pH excessively.
Dosing calculation: Each gallon of 12.5% liquid chlorine raises FC by approximately 1.5 ppm in 10,000 gallons of water. For a 20,000-gallon pool needing 20 ppm FC, you'd need about 13 gallons of liquid chlorine initially.
Test FC levels twice daily (morning and evening) and add liquid chlorine to maintain your shock level. Don't let FC drop below your target - this allows surviving algae to regrow rapidly.
Brush your pool walls and floor vigorously twice daily. Dead algae must be physically removed from surfaces where it can cling tenaciously. Use a stainless steel brush for concrete pools or nylon for vinyl/fiberglass.
Liquid chlorine raises pH, so you'll likely need muriatic acid to keep pH between 7.2-7.6. Add acid in small increments (1 quart per 10,000 gallons typically lowers pH by 0.2), wait 30 minutes, then retest.
Run your filter 24/7 during SLAM. Clean DE or cartridge filters daily - dead algae clogs filters rapidly. For sand filters, backwash when pressure rises 8-10 psi above clean pressure.
Vacuum algae debris to waste when possible to avoid recirculating dead material through your filter. If you must vacuum to the filter, clean it immediately afterward.
SLAM is complete when you pass the Overnight Chlorine Loss Test (OCLT):
The OCLT confirms no living algae remains to consume chlorine overnight.
Once SLAM is complete, reduce FC to normal levels (1-4 ppm depending on CYA). Balance remaining chemistry:
Warning: Never swim during SLAM. High chlorine levels can cause severe skin and eye irritation. Wait until FC drops to safe levels (typically under 5 ppm) before allowing swimming.
Store liquid chlorine in a cool, dry place away from other chemicals. Never mix different pool chemicals together.
Prevent future algae blooms by maintaining proper FC levels daily, keeping CYA between 30-50 ppm (70-80 for salt pools), and running your pump adequate hours for complete water turnover. Regular brushing and proper water balance prevent most algae problems before they start.
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