Black Algae in Pool And How to Get Rid of It

Do you see dark, rough spots on your pool walls or floor? That could be black algae—and it’s one of the worst pool problems you can have.

Black algae is one of the most difficult pool problems homeowners face. It has deep roots that grow into cracks and rough surfaces. Unlike green algae, it doesn’t simply float around. It anchors itself and forms tough, protective layers that are hard to scrub off.

Many pool owners don’t realize how harmful black algae can be. Inside its dark coating, it can carry bacteria that are dangerous to your health. Swimming in a pool with black algae may lead to skin irritation, eye problems, or even stomach issues if water is accidentally swallowed.

The problem also affects your pool’s structure. Black algae often stains and damages pool plaster, grout, and tiles. If left untreated, it can spread fast—costing more time and money to fix.

Normal cleaning won’t remove it, and even strong chlorine treatments sometimes fail if not done correctly.

In this blog, we’ll explain what black algae is, why it’s so hard to kill, and how to completely remove it step by step. You’ll also learn which tools and products from our store work best for the job.

What Is Black Algae, and Why Is It So Hard to Kill?

Black algae is not really algae. It’s more like a bacteria that looks like dark spots. It has a strong outer layer that protects it. That’s why it’s so hard to kill.

It grows in rough or shady spots in your pool. You’ll often see it in corners, in small cracks, or behind ladders. These areas don’t get much sunlight or water movement, which makes it easy for black algae to grow.

The worst part? Black algae has deep roots. Even if you clean the surface, the roots may still grow back. Normal brushing and chlorine don’t always work. That’s why you need a strong, step-by-step plan to remove it completely.

Is it Safe to Swim in a Pool with Black Algae?

No, it is not safe to swim in a pool with black algae. Here’s why:

1. Black Algae Can Carry Harmful Bacteria

Even though black algae looks like just dark spots, it can hide dangerous bacteria inside, like E. coli. These bacteria can cause skin infections, stomach illness, or even more serious problems.

2. It Means Your Pool Water Isn’t Healthy

If black algae is growing, it means your chlorine isn’t working right. That means other germs might be living in the water too. Your pool may look “okay,” but the water isn’t clean or balanced.

3. It Can Cause Skin or Eye Irritation

Swimming in a pool with black algae can lead to itchy skin, red eyes, or rashes, especially for kids or people with sensitive skin.

4. It Spreads Fast

Every time someone swims, they can accidentally spread the algae spores to other parts of the pool or even to other pools, hot tubs, or lakes.

How to Get Rid of Black Algae – Step-by-Step

You can get rid of black algae. But you need to act fast and follow each step carefully.

Step 1: Scrub the Spots Hard

First, you need to break the algae’s outer layer. This helps the chlorine and chemicals reach the roots.

Use a stiff pool brush. For concrete or gunite pools, use a stainless steel brush. For vinyl or fiberglass pools, use a nylon brush to avoid scratches.

Check out our best-selling pool brushes that are made for black algae scrubbing. They’re strong, safe, and easy to use.

Brush every black spot you see. Push hard, but don’t damage your pool. Scrub daily for best results.

Step 2: Shock the Pool – Strong Dose

Next, you need to shock the pool with a strong amount of chlorine.

Use double or even triple the normal dose. Black algae is tough, and it needs more chlorine to kill the roots.

Use a high-strength chlorine shock. It’s stronger than normal pool shock. Always follow the instructions on the bag.

For example:

• If your pool normally needs 1 pound of shock, use 2 to 3 pounds for black algae.

• Add shock in the evening for best results. Sunlight can make chlorine weaker.

We recommend our super shock treatment for tough algae jobs. It works fast and keeps water clear.

Step 3: Add Algaecide

After shocking, add a strong algaecide to the water. It gives extra help in killing any leftover algae.

Use a black algae-specific algaecide, if you can. It’s made to fight the deep roots.

If we don’t carry a black algae formula, try our strongest general algaecide. It can still help break down the algae.

Be sure to read the instructions and add the right amount for your pool size.

Step 4: Brush Again

Brushing once is not enough. After shocking and adding algaecide, brush all black spots again.

This helps remove dead algae and keeps the roots from growing back.

Brush daily for 3 to 5 days. Focus on cracks, corners, and steps. Don’t skip a day!

Step 5: Vacuum Out the Dead Algae

Now it’s time to clean the pool floor. Dead algae will settle at the bottom.

Use a manual vacuum with a head and hose. This gives you more control. If you want an easier option, use a robotic pool vacuum. It runs on its own and does a great job.

Step 6: Clean the Filter

Don’t forget your filter. It traps all the bad stuff in your pool, including black algae. If your filter is dirty, rinse it out well. If it’s old or worn out, replace it.

We recommend using a filter cleaner to wash away leftover algae and oils. Or check out our replacement filter cartridges for a fresh start.

A clean filter means better water and fewer problems.

How to Prevent Black Algae from Coming Back

Black algae is hard to get rid of. But the good news is—you can stop it from coming back if you take care of your pool the right way. Here's how:

Test Your Pool Water Every Week

Always check your pool water at least once a week. This helps you keep your chlorine, pH, and alkalinity levels in the safe zone. Use easy pool test strips or a digital test kit. If the water is off, fix it right away.

Keep Chlorine Levels Steady

Black algae loves pools with low chlorine. Make sure your chlorine stays in the right range:

• 1 to 3 parts per million (ppm) is best. If it drops too low, germs and algae can grow fast. Top up your chlorine tablets or shock the pool if needed.

Brush and Vacuum Weekly

Even if the pool looks clean, brush the walls, steps, and corners every week. Black algae often starts in rough or shady spots. Use a nylon brush for vinyl or fiberglass pools, and stainless steel for concrete or gunite.

Vacuum the pool to remove dirt where algae could grow.

Shock the Pool Often, After:

• A big pool party

• Heavy rain

• Very hot weather

• Or anytime the water looks cloudy or smells strange

This kills bacteria and keeps the water safe.

Check Problem Areas

Every week, look at:

• Steps

• Behind ladders

• Around lights

• Between tiles or in cracks

These are places where black algae hide. If you catch it early, it's easier to kill.

Some Products for Fighting Black Algae

• Stainless Steel Pool Brush – Great for deep scrubbing

• Chlorine Shock – High strength for killing algae fast

• Manual Pool Vacuum or Robotic Cleaner – Removes algae pieces

• Test Kits & Strips – Keep your water balanced and algae-free

• Filter Cleaner – Keeps your system clean after treatment

Buy Now

Conclusion

Don’t panic—black algae is tough, but not impossible to get rid of. With the right steps, tools, and a little patience, you can take back your pool.

Black algae comes back when pool care is skipped. So stay on top of your water tests, chlorine levels, and brushing.

Want the best tools to fight black algae? Check out our products—from strong pool brushes to chlorine shock and water test kits.

If this guide helped you, please share it with other pool owners or post it on social media. Let’s help everyone enjoy clean, safe water all season long!