Pool is green! How to Clear Algae from Your Above-Ground Pool and Restore Clear Water

A green, algae-filled pool can be alarming, but it’s fixable with the right steps. Different algae types (green, mustard/yellow, black) and different pool systems (chlorine, salt, mineral/bromine) require tailored approaches. First, test and balance your water – algae thrive when chlorine is low and pH is off. Ensure free chlorine is in the ideal range (1–3 ppm), pH around 7.2–7.6, and stabilizer (CYA) is sufficient. Adjust any imbalances before treatment. Run the pump continuously (at least 24 hours a day) to circulate chemicals throughout the water. Clean or backwash the filter now (and again later), since algae will quickly clog it.

Types of Pool Systems

• Chlorine Pools: Traditional above-ground pools use liquid or tablet chlorine. Shock treatments (high-dose chlorine) are the primary remedy for algae. In a chlorine pool you must monitor chlorine levels manually and may need extra sanitizers or algaecides when algae appear.

• Saltwater Pools: These generate chlorine via a salt cell. Treat algae much like a chlorine pool: brush, then “shock” by adding extra salt or liquid chlorine (even though chlorine is generated, you still raise the chlorine level). Keep your salt cell clean and salt level correct. Note: salt can corrode metal above-ground walls or ladders, so use corrosion-resistant equipment.

• Bromine/Mineral Systems: Bromine pools (more common in spas) kill bacteria more slowly than chlorine, so algae can persist unless treated with strong shock or specific bromine shock products. Mineral sanitizers (silver/copper) make water feel soft, and copper is an algaecide, but they cannot replace chlorine or bromine completely. You must supplement minerals with chlorine or bromine to kill algae. In any case, algae-removal steps (brushing, shocking, filtering) are similar regardless of sanitizer type.

Identifying Algae Types

• Green Algae: The most common. Free-floating “pea soup” water or slimy green film on surfaces. Easiest to kill with shock.

• Yellow/Mustard Algae: Yellowish-brown spots or patches, often on shaded walls or step corners. Chlorine-resistant and harder to kill. Requires repeated treatment and often a copper-based or specialty algaecide.

• Black Algae: Dark black spots or splotches, usually in one place (corners, seams, grout). Very stubborn; it roots into surfaces. Requires heavy shock and vigorous brushing (even acid washing in severe cases).

• Pink/Murky Algae (Bacteria Slime): Sometimes mistaken for algae, often bacterial. Brush and shock normally; a mild bleach soak (if pool empty) can remove it.

Algae Type	Appearance	Treatment Notes
Green	Bright green water or film	Easiest to kill. Brush and double-shock with chlorine.
Yellow/Mustard	Yellow patches on walls	Resistant; brush heavily and triple-shock. Use copper algaecide for stubborn spots.
Black	Dark black spots on surfaces	Hardest type. Requires very aggressive brushing and quadruple-dose shock. Often needs copper/silver algaecide and repeated treatments.

Step 1: Test and Balance Water

Before treating algae, test your water thoroughly. Algae flourish when sanitizer is low or pH/alkalinity is off. Use a test kit to measure free chlorine (or bromine/salt output), pH, total alkalinity (TA), and stabilizer (CYA). Adjust as follows:

• pH: Aim for 7.2–7.6. Alkaline water (>7.8) makes chlorine ineffective; too low pH (<7.0) can corrode equipment.

• Alkalinity: Keep TA around 80–120 ppm. Very high TA (cloudy water) or very low TA (pH swings) both hinder treatment. Use baking soda (sodium bicarbonate) to raise TA, or muriatic acid to lower it, if needed.

• Sanitizer: Ensure chlorine or bromine is present. If levels are zero or very low, algae already has an advantage. (In a salt pool, check salinity and cell function.) If free chlorine is low, add a small dose of sanitizer first, then proceed to shock.

  
  

• Stabilizer (CYA): If very low (<20 ppm), chlorine gets burned off by sun quickly; maintain ~30–50 ppm to protect chlorine.

  
  

  Balancing before shocking means your shock treatment will work more effectively. A balanced pool may still be green, but it’s the only state in which high-dose sanitizers can kill algae as intended.

Step 2: Boost Circulation and Clean Filter

Turn the pump on and keep it running continuously throughout the cleanup. Good circulation distributes chlorine and sucks algae into the filter. Run 24/7 if possible until the pool clears. Clean or backwash your filter immediately (and again later) because it will clog with dead algae and debris. Soak or rinse cartridge/sand filter media (some recommend a vinegar soak), or replace dirty cartridges. An extra-clean filter helps capture algae particles as your treatment proceeds.

Step 3: Remove Debris and Brush Everywhere

A vigorous scrubbing of walls and floor loosens algae so chlorine can kill it. First, use a skimmer net to remove leaves and large debris. Then brush the walls and bottom thoroughly with a pool brush (nylon bristles for vinyl/above-ground walls). Focus on corners, steps and crevices – algae love to hide there. You can add a paste of baking soda to your brush for a gentle abrasive scrub, which helps dislodge stubborn green/mustard algae without scratching the liner. As you brush, the water will usually turn very cloudy or green from the algae you’ve kicked up – this is normal. Scrub every inch; more brushing improves results, especially on black algae patches. (If you have mustard or black algae, you might need a stiff-bristled brush or even a scrubbing pad.)

If your pool has a vacuum system or skimmer vacuum port, hook up the vacuum hose and vacuum any loosened algae out of the water. In pools without a built-in skimmer, use a manual or battery-powered vacuum to suck out debris. This further reduces the algae load before chemical treatment. In short: brush first, then vacuum; this ensures chemicals kill algae, and physical removal gets them out of the water.

Step 4: Apply Shock and Algaecide

After thorough brushing, it’s time to shock the pool – raising chlorine (or sanitizer) to very high levels to kill algae. For a chlorine pool, use a high-quality pool shock (calcium hypochlorite or liquid chlorine). For salt pools, you can add bleach (liquid chlorine) or rely on a “superchlorination” mode if your generator has one. The key is to use extra-strong doses:

• Green algae: Use a double-strength shock. For example, add ~2 pounds of granular shock per 10,000 gallons (or ~2 gallons of liquid chlorine per 10,000 gal).

• Yellow/mustard algae: Use about triple the normal dose (e.g. ~3 lb shock per 10k gal). You may need to repeat this in 24 hours if algae persists.

• Black algae: Use about four times the normal dose (e.g. ~4 lb shock per 10k gal) and scrub vigorously. You might also use a copper- or silver-based algaecide at this time, which helps fight resistant algae. (Note: follow algaecide labels and pool chemical guidelines.)

Important: Always dilute shock in a bucket of water before adding (to avoid bleaching or gas pockets), and pour it in the deep end or around the perimeter for even distribution, not just through a skimmer. Perform shocking in the evening or at night, not midday, so sunlight doesn’t burn off the chlorine. Never swim during or immediately after shocking – wait at least 8 hours and until chlorine returns to normal.

If you use an algaecide, add it at the same time as the shock. For example, a copper-based or polyquat algaecide (often labeled for “green/yellow” algae) can aid the chlorine in collapsing algae cells. Always follow the product instructions and safety precautions.

Step 5: Let It Work and Filter Continuously

After shocking, keep the pump running at all times. Leave the filter on 24 hours if possible. The shock will kill the algae, and your filter will remove the dead particles. Within a few hours (or overnight), the water will usually turn a cloudy pale blue or gray – that’s dead algae suspended in water. Don’t panic; this means the shock is working.

If the water is extremely cloudy, you can use a pool clarifier or flocculant to help settle particles. A clarifier clusters tiny particles so the filter catches them, while a flocculant (use cautiously) makes debris sink to the bottom. Leave the filter on and give it 24 hours to pull out debris.

Step 6: Vacuum and Clean Again

Once the dead algae have settled (often by the next day), vacuum the pool thoroughly to waste/backwash. This means either vacuum through the filter’s waste line or backwash a sand filter, to avoid sending settled algae back into the pool. Remove as much sludge as possible. If using clarifier, vacuum out the bottom as debris settles.

After vacuuming, clean your filter one more time. Algae can stick inside filters, so disassemble and rinse or soak filter cartridges again, or backwash if you have a sand/DE filter. A freshly cleaned filter prepares for final balancing.

Step 7: Re-Test and Re-Balance

Now that your pool is (hopefully) clear, test the water again. High chlorine from shock may still linger, or it may have been used up. Check free chlorine/bromine, pH, alkalinity, and stabilizer. Adjust to normal ranges:

• Chlorine: 1–3 ppm for pools, 3–5 ppm for salt systems (as the chlorine generator output).

• pH: 7.2–7.6. If pH drifted during treatment, correct it.

• Total Alkalinity: 80–120 ppm.

• Cyanuric Acid: 30–50 ppm (if it dropped or was low).

  
  

Proper balance ensures the water stays clear and your sanitizer works. If the chlorine is still very high, wait for it to drop before swimming. Once balanced, your pool should be free of visible algae. Remove any residual debris and enjoy the clear water again.

DIY Remedies and Commercial Products

Besides standard shocks, you can use household items to help: baking soda is great for scrubbing algae spots (as a non-scratch abrasive) and for raising alkalinity. White vinegar (diluted) works for cleaning algae-stained surfaces or filters (though not in the pool!). Some use diluted bleach for a shock if pool-grade shock isn’t on hand (use 3% household bleach: ~8–10 gallons per 10,000 gal equals 1 ppm). Always be cautious mixing chemicals.

Commercial algaecides are formulated to speed up algae kill or prevent regrowth. Copper-based algaecides (“algaecide” or “copper stabilizer”) and polyquat algaecides (often called “green-to-clear” or “yellow-treat”) can be added after shock to kill lingering algae cells.

Clarifiers, flocculants and phosphate removers (to starve algae of nutrients) are also useful for future maintenance. Always read labels for above-ground pool safety and follow dosage instructions.

Common Mistakes to Avoid

• Skipping brushing or vacuuming. Some first-timers think chemicals alone will clear algae. In reality, you must scrub walls and vacuum dead algae. Simply shocking without brushing leaves algae stuck on surfaces.

• Adding shock improperly. Pouring granules directly can bleach liners or cause chemical hazards. Dissolve shock in a bucket (especially big doses), then pour it evenly around the pool. Do NOT dump a bunch into the skimmer, which can concentrate chemicals dangerously.

• Shocking at noon. Sunlight breaks down chlorine very quickly. Always shock in the evening so chlorine stays active overnight.

• Relying on the automatic vacuum alone. Robotic or suction cleaners help, but an algal bloom needs manual brushing and chemical treatment first. Many cleaners will just stir algae into the water if you haven’t killed it chemically.

• Not cleaning the filter. After shocking, filters get gummed with algae. A clogged filter will make water stay green or cloudy. Always clean or replace filter media before and after an algae treatment.

• Ignoring pH/alkalinity. If pH is high, even a big chlorine dose won’t work well. Always correct pH and TA first. Also, never try to adjust alkalinity right after shocking (wait 1–2 days as chlorine dissipates, otherwise you’ll burn through acid/baking soda).

• Swimming too soon. Don’t enter the pool until chlorine is safe (1–3 ppm) and the water is clear. Bacteria often accompany algae, so stay out until fully treated.

Keeping Water Crystal-Clear (Maintenance Tips)

• Maintain sanitizer levels: In summer, shock weekly and keep chlorine/bromine in range after busy weekends or heavy rain. For salt pools, check salt levels and cell output monthly.

• Regular brushing: Even with sanitizer up, algae can cling. Brush pool walls 1–2 times a week to prevent films from forming.

• Clean filter often: Backwash or rinse your filter monthly (more often in algae-prone seasons). A healthy filter is key to removing algae spores before they bloom.

• Remove debris promptly: Skim leaves/grass daily. Organic debris feeds algae.

• Use a pool cover: Covering your pool blocks sunlight and keeps out debris. Preventing algae spores (using a cover) makes infestation much less likely.

• Control nutrients: Algae feed on phosphates; use a phosphate remover if phosphate levels are high (test kits available). Also, avoid over-fertilizing nearby lawns or using pool floaties with compostable materials.

• Circulate water: Even when clean, run the pump 8–12 hours per day to keep water moving. Dead zones (no circulation) encourage algae.

• Monitor chemistry: Test water at least weekly (more in heat) and adjust. Using products like pH/stabilizer tablets or a baking soda additive (for alkalinity) can help keep chemistry in balance and reduce algae risk.

By following these steps methodically—identifying the algae, balancing chemistry, scrubbing/vacuuming, shocking, and filtering—you can turn a green, murky pool back to crystal clear. The process may take a day or two (sometimes longer for stubborn mustard/black algae), but consistency pays off. Soon your above-ground pool will be algae-free and ready for swimming again. Good luck, and see you poolside!