Pool Algae: How to Prevent It and What to Do When It Shows Up

Algae can turn a crystal-clear pool green overnight. It's one of the most common pool problems homeowners face — and one of the most preventable. Understanding what causes algae and how to respond quickly makes the difference between a one-day fix and a week-long battle.

Algae are microscopic plant-like organisms that thrive in warm, nutrient-rich water. They enter your pool through rain, wind, contaminated equipment, and even swimwear. When conditions are right — low chlorine, poor circulation, warm temperatures — they multiply rapidly.

There are three main types you'll encounter:

The most common type. Turns water green or cloudy and makes surfaces slippery. Can develop within hours when chlorine drops. Relatively easy to treat when caught early.

Appears as yellow or brownish patches on walls and floors, often in shaded areas. Frequently mistaken for dirt or sand. More resistant to chlorine than green algae and tends to return if not treated aggressively.

The most stubborn type. Appears as dark spots with a protective outer layer that shields it from chlorine. Roots itself into plaster and grout. Requires aggressive brushing and repeated treatment to eliminate.

Algae blooms almost always trace back to one or more of these conditions:

• Low free chlorine — the most common cause; chlorine is what kills algae before it can establish
• High pH — chlorine becomes significantly less effective above pH 7.6
• Poor circulation — dead spots in the pool where water doesn't move allow algae to take hold
• Warm water — algae grows faster as water temperature rises
• High phosphates — phosphates from leaves, fertilizer runoff, and some pool chemicals act as algae food
• Inadequate filtration — a dirty or undersized filter can't remove algae spores fast enough

Prevention is far easier than treatment. Here's what works:

Maintain proper chlorine levels. Keep free chlorine between 1–3 ppm consistently. In hot weather or after heavy use, test more frequently — chlorine can drop to zero in a matter of hours.

Keep pH in range. Target 7.4–7.6. High pH dramatically reduces chlorine effectiveness and creates ideal conditions for algae.

Shock regularly. A weekly or bi-weekly maintenance shock destroys algae spores before they can establish. Shock after heavy rain, heavy use, or any time chlorine drops significantly.

Run your pump long enough. Your pool water should turn over completely at least once every 8 hours. In summer, running the pump 10–12 hours per day is often necessary.

Brush weekly. Brushing walls, steps, and the floor disrupts algae before it can attach and root. Pay special attention to shaded areas and corners.

Use an algaecide as a preventive. A weekly dose of a quality algaecide — especially a polyquat algaecide — adds a layer of protection during peak algae season.

Keep phosphates low. Use a phosphate remover if testing shows elevated levels, especially if you have trees or landscaping near the pool.

1. Test and adjust pH to 7.2–7.4 before adding any chemicals.
2. Brush the entire pool — walls, floor, steps — to break up algae colonies.
3. Shock aggressively. Use 2–3 lbs of calcium hypochlorite per 10,000 gallons for a moderate bloom; up to 4 lbs for severe cases.
4. Add an algaecide after shocking (wait until chlorine drops below 5 ppm to avoid degrading the algaecide).
5. Run the filter continuously for 24–48 hours.
6. Vacuum to waste — bypass the filter to remove dead algae directly out of the pool rather than recirculating it.
7. Retest and balance chemistry once water clears.

Yellow algae requires a more aggressive approach:

• Wash all swimwear and pool equipment that may have been in contact with the pool
• Brush affected areas thoroughly
• Triple-shock the pool (3 lbs per 10,000 gallons)
• Use a yellow algae-specific algaecide
• Vacuum to waste after 24 hours

Black algae is the hardest to eliminate:

• Use a stainless steel brush to physically break through the protective layer on each spot — this is essential
• Apply a black algae-specific algaecide directly to spots using a sock or applicator
• Shock at 3–4 lbs per 10,000 gallons
• Repeat brushing and treatment every 3–4 days until spots are gone
• Black algae in plaster may require professional acid washing if it keeps returning

If your pool is completely green and you can't see the bottom, don't just dump in chemicals and hope for the best. The process takes time:

1. Remove large debris with a net — don't vacuum yet
2. Test and adjust pH
3. Shock heavily and run the filter
4. Allow 24–48 hours for the water to clear enough to see
5. Vacuum to waste once you can see the bottom
6. Backwash the filter frequently during treatment — it will clog quickly with dead algae

Expect the process to take 3–5 days for a severe bloom.

If algae keeps coming back despite proper treatment, there's usually an underlying issue — a filtration problem, a circulation dead spot, or a persistent phosphate source. Our team can diagnose the root cause and get your pool back to clear.

Contact us to schedule a service visit in Western Massachusetts or Northern Connecticut.