Black Algae in the Pool! The Best Tips for Removal and Prevention

Published Categorized as Pool Care
Dirty swimming pool with leaves in the pool bottom. Cleaning concept

Wear gloves and some protective layers right now. We’re about to get down and dirty to get rid of the black algae in pool walls!

The Big Picture: What Is Black Algae?

To better explain what it is, it will be easier to explain what it is NOT:

  • It is not to be confused with algae of green color that closely resemble seaweeds, and
  • It is not like plants that undergo photosynthesis to make food

Instead, black algae are cyanobacteria.

Like bacteria, they are living organisms that naturally look like blue-green algae.

However, they are called black algae because of their reactions with other WATER-SOLUBLE PIGMENTS in the long run.

We’ll talk more about black algae in this article, so read ahead if you want to learn more!

Is Black Algae Harmful?

Yes, blue-green algae such as cyanobacteria can be dangerous. Often seen inhabiting natural bodies of water, cyanobacteria also release cyanotoxins, natural poisons.

Drinking and exposure to black algae-infected pool water can cause severe conditions of the following:

  • Nausea
  • Stomach cramps
  • Liver damage

Additionally, they attract insects or harbor harmful organisms, contributing to further contamination of pools.

Thus, you must be able to identify black algae as soon as it appears and take immediate action to get rid of it.

How Does Black Algae Grow in Pools?

There are three ways through which this can happen:


Since black algae grow in a natural body of water, pool infestation usually occurs due to cross-contamination.

If SOMEONE SWAM IN A LAKE WITH BLACK ALGAE, that person can bring it to your pool using the same swimming suits and equipment.

Airborne Transfer

Black algae can also enter pools via airborne spores.

However, this is less likely to happen.

Environment Conditions

More importantly, black or blue-green algae prefer thriving in pools with the following conditions:

  • High pH level
  • Low chlorine level

This will be amplified, especially if you are located in warmer climates.

You should always make sure that your pools are properly balanced.

Here’s How to Get Rid of Black Algae in Your Pool!

The harmful bacteria in your backyard pool is fortunately NOT impossible to kill. That knowledge should be good enough to avoid such a health risk at home!

Let’s start going through the step-by-step process of killing your pool’s black algae, shall we?

What You Will Need

  • Pool Filter Cleaner
  • Pool Algae Brush
  • Pool Water Test Strips
  • Chlorine Tablets
  • Calcium Hypochlorite Shock
  • Putty Knife or Scraper
  • Safety Goggles
  • Chemical-Resistant Gloves
  • Backwash Hose

Depending on your pool landscape and condition, you may also want to consider preparing filter sand and replacement filter cartridges.

Remember: preparation is IMPORTANT to ensure seamless transitions between steps.

Let’s begin!

What You Will Do

These are the steps to do if you want to properly remove black algae in your pool!

Step 1: Clean the Pool Filter

There are TWO WAYS through which you can approach this step:

  1. Cleaning and backwashing your filter, or
  2. Replacing the filter altogether

The choice will depend on factors concerning the EXTENSIVENESS of your algae problem.

A simple wash can do the trick for small amounts of black algae in the pool.

Otherwise, pools with an extensive black algae problem can benefit from a good FILTER CLEANER.

Regardless, full replacement of the pool filter is the better option should budget not be a constraint.

Step 2: Test the Water Chemistry of the Pool

Use your test strips to learn more about the condition of the black algae in the pool.

These are the optimal levels we want our swimming pool to be at:

  • pH Levels: 7.4 to 7.6 (adjust using pool pH increaser or reducer)
  • Alkaline Levels: 100 ppm to 150 ppm (adjust using alkaline increaser or reducer)
  • Chlorine Levels: 2 ppm to 5 ppm (adjust using chlorine tablets)
  • Stabilizer Levels: 30 to 50 ppm (add cyanuric acid to increase and dilute to decrease)

Determining the water chemistry of your pool is important because of TWO REASONS:

  • Safer use of swimming pools
  • Preparation for future pool shock

Maintain your pool chemistry to avoid black algae growth and health risks.

Adjust them to their optimal levels THIS early on in the procedure. Doing so will help ease the manual work needed to keep the pool clean in future steps.

Step 3: Brush and Remove the Black Algae From the Pool Walls

Get yourself a pool brush and give your pool’s walls a good scrubbing.

This step is meant to loosen the black algae from the pool surfaces so you can kill them off during the shocking process later on.

Specific types of brushes are recommended depending on the TYPE OF POOL that you have:

  • Stainless steel brush for concrete pools
  • Nylon bristled brush for fiberglass or vinyl liner pools

A regular pool brush can also work on pools not infested by black algae that much, but the nylon brush is the better option.

Otherwise, you would need to couple your brushes with good elbow grease. Doing so will help you get into each crack and crevice of your swimming pool walls.

Step 4: Scrape the Black Algae Spots From the Pool Walls

This step will ask you to remove the remaining black algae on your pool walls.

The following tools are highly recommended for you to use:

  • Putty knife
  • Scraper
  • Pumice stone
  • Wire brush

This step is necessary because you are expected to NOT remove every black algae spot in one go.

Brushing should have loosened them up, and now SCRAPING should allow you to remove them fully.

Step 5: Scrub the Black Algae Patches With Granular Chlorine

It would be best if you addressed the roots of your black algae.

Directly rubbing CHLORINE on the black algae spots post-scrubbing and brushing should do the trick!

Here’s a quick protocol to follow:

  1. Make sure that you are wearing your chemical-resistant gloves and protective eyewear.
  2. Break a granular chlorine tablet in half.
  3. Use each half to scrub the black algae spots.

Get yourself a chlorine tablet holder and a pool pole if you’re having difficulty reaching the edges of your swimming pool.

Regardless, DO YOUR BEST to apply chlorine directly onto the affected pool surfaces.

Step 6: Triple Shock the Pool With Calcium Hypochlorite

Shocking the pool will kill black algae that are now FLOATING on your pool water.

However, one dose of shocking is NOT ENOUGH if you’re dealing with black algae.

You need to INCREASE the dosage to at least three times the regular amount of calcium hypochlorite.

Here are three good rules of thumb to follow:

  1. Amount needed to triple shock: 3 pounds of calcium hypochlorite per 10,000 gallons of water
  2. When to shock the swimming pool: At night so that the sun’s UV rays will not deplete it
  3. What to leave in the swimming pool water during shocking: Pool equipment such as pool brush, tablet holder, and leaf skimmer

You can also leave any pool toys during shocking. This will address BLACK ALGAE SPORES that have likely been attached to nearby pool accessories.

Leave any equipment near the pool’s shallow end to sanitize them together as you go through the shocking process.

Hand holding white chlorine tablets over swimming pool skimmer. Chlorination of water in pool for disinfection and prevention against the development of microbes.

Step 7: Run the Pump

Run the pump for a good 24 hours.

You will need this to fully expose your pool and the black algae to the shock treatment!

Afterward, you can fully expect the following scenarios:

  • Cloudy and musty pool water
  • Dirty and black algae-infested filter

These are perfectly normal results to come back to.

The pool may look EVEN DIRTIER now, but you have nothing to worry about because the next steps will address this!

Running the pump is a vital step in the process since it will effectively disperse the shock to kill the black algae in your pool.

Step 7.1 Pool Professional Tip: Brush the Pool Walls Again

While the pump is running, do your best to intermittently brush the walls again!

Take advantage of the high chlorine levels and brush AT LEAST TWO MORE TIMES.

Step 8: Filter, Shock, Pump, and Brush the Pool Again

Be prepared to review the previous steps you have already done for this step.

We’ll be doing this, so your swimming pools get the cleaning they rightfully deserve after being infested with black algae!

Allow us to guide you through this step-by-step once more, shall we?

Step 8.1: Clean the Pool Filter Again

Yes, you cleaned this before in Step 1.

However, cleaning it this time will be because of the accumulated bits of black algae resulting from the chlorine treatment and running pump!

Some reminders to keep in mind:

  • Use a filter cleaner to get to the bottom of the black algae infestation in your pool.
  • There is NO NEED to replace the filter cartridges if you’ve done this in Step 1. If you haven’t yet, you may CONSIDER DOING SO right now.
  • Otherwise, backwashing the sand filters or rinsing the cartridges can trick.
Step 8.2: Shock the Pool Again

If there are still remnants of black algae in your pool, it would be best to SHOCK THE POOL AGAIN.

These configurations should be enough for the second round of shock treatment:

  • Amount needed to triple shock: 2 pounds of calcium hypochlorite per 10,000 gallons of water
  • When to shock the swimming pool: At night so that the sun’s UV rays will not deplete it
Step 8.3: Run the Pump Again

Once again, after shocking the pool, you should run the pool pump to fully spread the treatment.

You can run this any time from 12 to 24 hours.

Step 8.4: Brush the Pool Again

Don’t forget to BRUSH THE WALLS OF THE POOL once more after running the pump!

Allow all the black algae to reach the proper filtration system of your pool.

Under no circumstances should you skip this step to ensure that NO DEEP ROOTS will be left in your pool before eventual use.

Step 9: Test the Water Chemistry of the Pool Again

More than the drastic improvement of the pool water visually, a good indicator of your pool’s improved health is its WATER CHEMISTRY.

Remember Step 2? You have to recreate that test again.

For faster reference, the following indicators should result in the following by now:

  • pH Levels: 7.4 to 7.6
  • Alkaline Level: 100 ppm to 150 ppm
  • Chlorine Level: 2 ppm to 5 ppm
  • Stabilizer Level: 30 to 50 ppm

Adjust the levels as needed.

Once everything is of normal value, you should be able to say that you were able to properly treat black algae in your pool!

The next challenge would be to PREVENT BLACK ALGAE from ever returning to your pool’s surfaces.

Step 10: Observe Abnormal Growth

This is more of a precautionary step than an action.

All you need to do is keep an eye on your growth for any black algae growth. But you’ve done Steps 1-9 diligently. Then how could they ever return?

There are two simple answers to this:

  1. You have missed some roots in hard to reach crevices of the pool, or
  2. You induced the black algae back to the pool using infected equipment

Don’t worry because these are all NORMAL!

Should this happen, immediately address it by once again brushing, scrubbing, and shocking your pool.

If this is too tiresome for you to go through again, it may be time to finally hire a pool professional to help you with the algae bloom.

Identifying Black Algae in the Pool: How Do You Do It?

If your pool is starting to look like it harbors mold, you’re probably looking at black algae.

Please do NOT confuse them with mineral stains, as they also cause black stains on the pool walls.

Instead, here’s a list of VISUAL CUES that you can keep an eye out for to properly identify black algae in your pool:

  1. Black or blue-green spots with raised heads
  2. Attached to the pool surfaces, especially crowding in rough areas
  3. Clumped and don’t float freely in the water

Going into the location specifics, black algae have their habitat preferences.

Black algae prefer porous surfaces they can latch onto to settle their roots down:

  • Concrete walls
  • Pool plaster
  • Gunite surface

Pools made of fiberglass or vinyl liners won’t usually find black algae on their pool’s surfaces.

However, the possibility will always be there.

Black algae are generally harder to scrape or brush off the pool surface.

You should do your best in the MAINTENANCE OF YOUR POOL.

Start Cleaning Your Pools Now!

Recognizing the existence of black algae in your pools is the FIRST STEP for you to prevent black algae from repeated growth in your pool.

Don’t forget the important methods, indicators, and equipment that you’ll be employing a lot throughout the entire cleaning process:

  • Cleaning the Filter – backwashing or cartridge replacement
  • Testing the Water Chemistry of the Pool Water – pH level, alkaline level, chlorine level, stabilizer level
  • Brushing and Scraping – pool brush, nylon brush, scraping tools
  • Scrubbing – chlorine tablets
  • Shocking the Pool – calcium hypochlorite
  • Pumping the Pool – water pool pump

Employ proper pool care and get rid of the black algae heads on your pool’s surface in NO TIME AT ALL.

Approach your manufacturer’s instructions about the equipment and materials you’ll be using.

By Christine Newton

Hi, I’m Christine. I’m a swimmer and the Lead Editor of YourPool. I’ve been working with content for as long as I can remember, and I really enjoy finding inspiration for our readers. Right now, I’m living in a small pool (literally), but my dream is to have a complete oasis one day. Stay tuned—I’ll make it happen!

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