Why has my pool turned black?

A black pool is never a welcome sight for any pool owner. After spending time and money trying to keep your pool sparkling clean, seeing it turn black can be frustrating and concerning. But don’t worry, there are some common reasons why pool water can turn black, and solutions to get your pool back to its original beautiful blue.

What causes pool water to turn black?

There are a few primary causes of black pool water:

  • Algae growth
  • Problems with the filter and filtration system
  • Chemical imbalances
  • Metal stains and deposits
  • Organic contaminants like leaves and dirt

Let’s explore each of these common culprits behind black pool water:


Algae is one of the most common causes of black water in pools. When algae begins to rapidly spread in your pool, it can turn the water a dark green, black, or brown color.

There are a few different types of algae that can thrive in pools:

  • Green algae – Most common type, grows on walls and floor
  • Black algae – Also called black spot algae, dots pool walls
  • Mustard algae – Yellow-green color, coats pool floor
  • Pink algae – Reddish-pink, sticks to walls and objects

Algae needs sunlight, nutrients, and water to grow. It spreads rapidly, fed by phosphates and nitrogen in the water. Warm, stagnant water is the perfect breeding ground for algae.

Problems with Filter and Filtration

Your pool’s filter system plays a critical role in keeping water clean and clear. When the filter isn’t working properly, contamination and cloudiness can occur. Here are some common filter issues that can cause black water:

  • Dirty filter – A clogged, dirty filter can’t trap debris and algae properly.
  • Improper filter settings – If the filter valve is incorrectly set, water won’t flow properly for cleaning.
  • Leaking filter – Cracks in the filter or loose fittings allow unfiltered water to bypass into the pool.
  • Old filter – Over time, the filter media becomes too clogged and worn out to catch contaminants.
  • Incorrect pump speed – The pump must run at the right flow rate to push water through the filter for cleaning.

Chemical Imbalances

To keep water clean and algae-free, your pool’s chemical balance needs to be properly maintained. The main chemicals to monitor and balance are:

  • Chlorine – Sanitizes water and kills algae and bacteria. Too low allows contanimants. Too high irritates skin and fades suits.
  • pH – Measure of alkalinity/acidity. Out of range pH reduces chlorine effectiveness.
  • Alkalinity – Buffers pH changes. Too low causes pH bounce, staining and corrosion.
  • Cyanuric Acid – Stabilizes and protects chlorine from UV rays. Too high causes cloudy water.

When any of these chemical levels fall out of the ideal range, it can allow algae growth and cloudy black water. Routine testing and balancing is key for clear water.

Metal Stains and Deposits

Dissolved metals like copper, iron, and manganese can stain pool surfaces and cause dark, cloudy water. These undesirable metals come from the source water or corrosion of equipment and surfaces. Other sources include:

  • Metal items in pool – ladders, drains, fittings, rails
  • Improperly balanced water
  • Harsh pool chemicals

Metal stains start appearing in small dark dots and splotches. Soon they spread, turning water black, brown, or green. Removing built-up metals requires a stain remover or chelating agent.

Organic Contaminants

As dirt, leaves, grass, pollen and other organic matter build up in your pool, the water can start turning cloudy and black. Materials like leaves introduce tannins and organic dyes into the water.

Surface debris also gives algae food to grow on. Proper skimming and cleaning is essential to get rid of organics before they degrade water quality and color.

How to Treat Black Pool Water

Now that you know what causes pool water to look black, here are the general steps to treat it:

1. Test and Balance Chemical Levels

Using high quality test strips, test and record current chemical readings for pH, alkalinity, chlorine, and cyanuric acid. Make any needed adjustments to bring levels back into the ideal ranges. Properly balanced water inhibits algae growth and optimizes filtration.

2. Clean or Replace the Filter

Check your pool filter and clean or replace it if needed. Refer to the filter manual for how to remove buildup, soak, or replace old media. A clean, fully functioning filter will remove contaminants.

3. Shock the Pool Water

Shocking the pool oxidizes contaminants and kills algae and bacteria. Using a strong chlorine shock at a high dose cleans the water. Shocking may need to be repeated over several days for severe algae cases.

4. Brush Walls and Floor

Use a stiff pool brush to vigorously scrub the walls, floor, and all surfaces to remove stubborn algae. This prevents it from gaining a new foothold when you refill the pool.

5. Vacuum to Waste

Vacuuming all debris to waste removes settled particulates from the pool floor. This prevents them from being stirred up when the pool is refilled.

6. Clean or Replace Filter Again

After vacuuming, clean or replace the filter one more time to ensure it is free of all remnants that could seed algae regrowth.

7. Add Algae Preventative

After the initial cleanup, add an algaecide like polyquat 60 as a preventative measure against future algae growth.

8. Time Filter Run

Run the filter continuously for at least 24 hours. This circulates and filters the water multiple times to capture any particles that could remain.

9. Test and Adjust Chemistry

Test pH, alkalinity, and sanitizer levels again. Make final minor adjustments to ensure water balance is ideal after treatment.

10. Add Clarifier

Once water is clean, add a clarifier or water polisher to group together tiny particles so they can be better filtered out. This helps restore sparkling clarity.

Preventing Black Algae in the Future

Once you get rid of black algae, there are some key prevention measures to implement so it doesn’t return:

  • Maintain proper chlorine level.
  • Regularly test and balance pH and alkalinity.
  • Shock weekly to oxidize contaminants.
  • Closely monitor cyanuric acid level.
  • Run filter enough hours per day to turnover water 2-3 times.
  • Frequently brush walls and skim surface.
  • Clean pump basket and filter regularly.
  • Add algaecide with start up at beginning of season.
  • Use water clarifier to prevent gray water conditions.

Also, check for common maintenance issues that can set the stage for algae growth:

  • Low water level
  • Faulty valve or broken return fitting
  • Cracks in plaster
  • Damaged filter components
  • Worn out media needing replacement

Fixing these mechanical defects denies algae the environmental edge it looks for.

When to Call a Pool Service for Black Algae

Severe black algae may require the help of a professional pool service company. Consider calling in the experts if:

  • Algae has overtaken more than half the pool.
  • Filter cleaner and shock treatments aren’t restoring clarity.
  • Water remains cloudy and black after initial treatment.
  • Algae recurs shortly after cleaning.
  • Unsure about proper use of chemicals for treatment.
  • Other equipment problems are suspected.

For stubborn black algae cases or underlying issues, a pool pro has the tools, chemicals, and experience to get rid of it for good. They can also diagnose and correct any maintenance problems contributing to algae growth.

FAQs on Black Pool Algae

Why did my pool water turn black overnight?

Pool water can turn black quickly due to “black algae” (also called black spot algae). This dark algae grows in dots and spreads rapidly overnight under the right conditions. Ensure proper sanitizer levels and circulation to prevent overnight algae takeovers.

Can you swim in a black pool?

No, you should avoid swimming in black, contaminated pool water as it contains visible algae. Algae can cause skin irritation, rashes, and illness if ingested while swimming. The pool needs to be treated to kill the algae and make the water swimmable again.

How long does it take to clear up black algae?

With proper treatment, black algae can usually be cleared up in 5-7 days. Severe cases may take up to 2 weeks to fully treat. Be patient, as shocking and vacuuming alone won’t immediately make the water clear. It takes time for the dead algae and particles to be filtered out.

Can you vacuum during algae treatment?

Yes, you can and should vacuum the pool walls and floor at least once during black algae treatment. This is done after an initial heavy dose of shock is added. Vacuuming removes algae spores and debris so they aren’t stirred up when the pool refills.

How do you maintain a pool after algae?

Once black algae is gone, maintain the pool by:

  • Keeping chlorine 1-3 ppm and pH 7.2-7.6
  • Shocking weekly
  • Cleaning filter monthly
  • Testing water 2-3 times a week
  • Brushing walls and skimming often
  • Adding clarifier and algaecide

Consistent maintenance prevents a repeat algae outbreak.


Seeing your pool water turn black can certainly be troubling. However, by understanding the common causes like algae growth and filter issues, black water can be successfully treated. With prompt troubleshooting, proper sanitization, filtration, and consistent maintenance, your pool can soon be restored to a clear, sparkling oasis again.

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