Free Guide to Clearing Green Pool Water

Understanding Why Pools Turn Green and What Causes It

A green pool typically results from algae growth, which thrives when pool chemistry becomes unbalanced. Algae are microscopic plants that float in water and multiply rapidly under the right conditions. When chlorine levels drop below 1 part per million (ppm), algae can begin to establish themselves. The color you see—ranging from light green to dark green or even black—depends on the algae type and how long it has been growing.

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Several factors contribute to algae blooms in residential pools. Sunlight provides energy for algae growth, which is why pools in sunny climates or during summer months experience more problems. Warm water temperatures between 70°F and 90°F create ideal conditions for algae reproduction. Heavy rainfall can introduce contaminants and dilute chlorine concentration. Organic debris like leaves, pollen, and grass clippings introduce phosphates and nitrates that feed algae.

Insufficient filtration allows algae particles to remain suspended in water rather than being trapped and removed. When circulation pumps run for fewer than 8-10 hours daily, water doesn't cycle through the filter enough times to maintain clarity. Dead spots in the pool where water doesn't circulate well—like corners and areas behind equipment—become breeding grounds for algae.

Improper pH balance also contributes to algae growth. When pH drops below 7.0 (acidic), chlorine becomes less effective at killing algae. When pH rises above 7.6 (alkaline), chlorine efficiency decreases similarly. Understanding these causes helps you address the root problem rather than just treating symptoms. A green pool is actually a sign that water chemistry has shifted out of safe parameters.

Practical Takeaway: Before beginning any treatment, recognize that algae growth indicates a chemistry imbalance. Simply adding chemicals without understanding the cause means the problem may return once treatment ends. Document what conditions preceded the algae bloom—weather patterns, filtration runtime, recent rainfall—to prevent recurrence.

Testing Your Pool Water and Reading Chemical Levels

Accurate water testing is the foundation of clearing a green pool. You cannot treat what you cannot measure. Pool test kits come in three main types: liquid reagent kits, test strips, and digital monitors. Liquid test kits, like the Taylor K-2006 or AquaChek, cost between $30-$60 and are considered more accurate for determining chlorine, pH, alkalinity, and acid demand. Test strips cost $15-$25 for a container of 50-100 strips and provide results within seconds, though they're slightly less precise. Digital testers range from $100-$500 and offer precise readings but require calibration.

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For green pool recovery, you'll need to test four key parameters. Free chlorine should be 1-3 ppm during normal conditions, but treating green water requires 10-20 ppm. Total chlorine includes both free chlorine (which kills algae) and combined chlorine (which indicates chlorine has already been used). pH should stay between 7.2-7.6 for chlorine to work effectively. Total alkalinity, the pool's resistance to pH change, should measure between 80-120 ppm. During green pool treatment, you may also test for cyanuric acid (stabilizer), which should be 30-50 ppm.

Test water properly by collecting samples from at least 12-18 inches below the surface, away from the return jets where fresh chemicals enter. Samples taken from the surface or near circulation areas give misleading readings. Use clean collection bottles—never reuse containers that held chemicals. Follow kit instructions precisely, as timing matters significantly. For example, some liquid tests require waiting exactly 20 seconds before reading results; timing off by 5 seconds can produce different readings.

Test your pool at the same time each day, ideally early morning before the sun begins degrading chlorine. Record all readings in a log—noting the date, time, weather conditions, and any chemicals added. This creates a historical record showing whether levels are improving or worsening, which guides your next steps. Many pool owners find that weekly or twice-weekly testing during green water recovery provides better decision-making than sporadic testing.

Practical Takeaway: Invest in a liquid test kit rather than relying on test strips alone. The $30-$50 cost is worthwhile for accuracy during green pool recovery. Document all readings. Your chemical log becomes a record of what worked and what didn't, helping you prevent future algae blooms.

The Step-by-Step Process for Treating Green Pool Water

Clearing green water involves a systematic sequence: brush the pool, raise chlorine levels, run the filter continuously, and maintain chemistry until clarity returns. This process typically takes 24-72 hours for light-to-moderate green water, and 5-10 days for heavily green pools.

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Begin by brushing the pool walls, floor, and steps vigorously with a stiff-bristled pool brush. This agitates algae on surfaces, making them suspend in the water where chlorine and filtration can address them. Spend 15-20 minutes thoroughly brushing, focusing especially on corners and the deep end. Brushing alone won't clear the pool, but skipping this step means algae cling to surfaces and resist chemical treatment. Follow brushing by vacuuming to waste—using a multiport valve to send debris directly to the drain rather than through the filter, since a clogged filter reduces circulation.

Next, shock the pool with calcium hypochlorite or sodium hypochlorite shock. "Shocking" means adding high doses of chlorine to quickly kill algae. For green pools, you may need 2-4 times the normal shock dose—calculated by multiplying your pool volume by the shock amount per 1,000 gallons. A pool containing 15,000 gallons that normally requires 2 pounds of shock per 10,000 gallons might need 6-12 pounds for green water. Follow shock product instructions for your specific brand and pool volume. Add shock in the evening when the sun won't degrade it as quickly, distributing it evenly across the pool surface rather than dumping it in one spot.

After shocking, run the circulation pump and filter continuously—24 hours per day if possible. The filter removes dead algae particles, and continuous circulation prevents water from stratifying (layers of different temperatures and chemistry). If your pump has a timer, disable it temporarily. Some pool owners run pumps continuously for 48-72 hours until water clarity visibly improves, then return to normal 8-10 hour filtration schedules.

Monitor chemistry daily. When chlorine levels drop below 10 ppm, add more shock. When pH creeps above 7.8, add pH decreaser. As algae dies and water clears, you'll see white dust—dead algae particles—which the filter traps. When the dust stops appearing and you can see the bottom drain, the pool is nearly clear.

Practical Takeaway: Don't add all chemicals at once and hope for results. The brushing-shock-filter-monitor cycle requires patience and repeated testing over several days. Green pool recovery is a process, not an instant fix.

Managing Filter Performance During Green Pool Treatment

Your filter is central to clearing green water, but algae-clogged filters lose efficiency. Before treatment begins, clean the filter thoroughly. For sand filters, backwash by switching the multiport valve to "backwash," running the pump for 2-3 minutes until the sight glass shows clear water, then switching to "rinse" for one minute. For cartridge filters, turn off the pump, remove cartridges, and rinse them with a garden hose, spraying from top to bottom to flush out particles. For diatomaceous earth (DE) filters, backwash and add fresh DE powder as instructed.

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During green pool treatment, monitor filter pressure using the pressure gauge mounted on top. Normal operating pressure varies by filter type—sand filters typically run 15-20 psi, cartridge filters 8-15 psi. When pressure rises 8-10 psi above normal, the filter needs cleaning. Green pools may require cleaning the filter every 24 hours during treatment, versus weekly during normal operation. Ignore the filter cleaning frequency on your timer; green pools demand more frequent attention.

Some pool owners choose to run the filter on multiport valve "waste" setting during green pool treatment rather than "filter." This sends