How To Remove Algae Stains From Pool Walls & Bottom?

Algae. What’s the big deal? It’s just a plant, right? WRONG. 

Have you seen what algae can do to a pool? It’s disgusting! You don’t want to go swimming in that, do you?

Well, I don’t blame you. So how can you remove algae stains from the walls and bottom of your pool?

Follow these steps:

  • Test the pool (pH, alkalinity, etc)
  • Identify what type of algae problem you have
  • Balance the pool
  • Shock the pool
  • Add algaecide
  • Brush pool
  • Run the pump

Ready to get started? Keep reading!

Determine the algae stain’s cause

algae stains pool bottom

Several things can cause Algae stains in a swimming pool, but determining the cause is the first step in getting rid of them.

There are three main ways to determine what’s causing your algae stains:

  1. Visual inspection
  2. Testing the water

Both of these methods will help you isolate whether or not algae is the culprit.

Visual inspection

If you’re not sure if there are algae in your pool, you can do a simple visual inspection to determine if it is present. The most common algae will be a greenish hue and will be slimy to the touch.

Algae can also be black, mustard, or additional colors.

If you determine there is no sign of algae, you may want to seek out other causes for your discoloration, such as metals like copper or mercury, decaying organic matter (leaves), or dirt.

Identifying algae in the pool with testing kits

Professional pool testing kits are an excellent way to determine what kind of algae you have in your pool, and they will give you some tools for removing it. 

The most common form of testing is a phosphate test kit, which measures the phosphate level in the water. Algae feed on phosphates, so if there’s a significant amount in your pool, it’s probably responsible for your algae growth.

pool parameters testing kit

You can also use a more comprehensive test. The five most common parameters are pH, total alkalinity, calcium hardness, Chlorine, and stabilizer (cyanuric acid).

For algae, specifically, you will want to pay special attention to pH levels and chlorine levels.

pH is the measure of how acidic or basic a substance is. It ranges from 0 (most acidic) to 14 (most basic). The pH should be between 7 and 8 to provide swimmers with a safe, healthy environment.

On the other hand, Chlorine is a sanitizer that kills algae and other organisms in the water.

If you see algae stains, it’s a good idea to confirm your chlorine levels aren’t too low. The recommended level of Chlorine for a swimming pool is 1-3 parts per million (ppm).

How do I get rid of algae from the pool?

Algae thrive in any pool. When the conditions are right, they may develop into a visible colony.

Here are the reasons algae grow in the first place:

  • Poor water circulation; Water circulation is essential for keeping the pool clean and healthy.
  • Poor water balance; water balance (pH, alkalinity, calcium, and cyanuric levels) must be monitored and maintained to protect the pool from bacteria and other contaminants
  • Sanitation is also crucial. For optimal sanitation, please maintain consistent chlorine levels
  • Poor water filtration; short filter run times, or an ineffective filter

Balance the pH

To keep your pool water clean and healthy, it is crucial that the pH level remains within an optimal range. 

Algae thrive when there’s too much acidity or base for them; therefore, you should aim to maintain a balance between 7-8 on either side (acidic vs alkaline).

If your pH is out of balance, you can use a pH increaser or decreaser to adjust it. Be sure to check your chlorine levels as well, as they should be between 1 and 3 ppm for the best results.

Adjusting the Chlorine is often enough to kill algae in most pools.

Use an algaecide

Algaecides kill or repel algae. They come in the form of liquid tablets and are applied through your pool’s skimmer.

If you have a vinyl liner pool, it’s best to use an algaecide that is meant for vinyl liner pools, as some algaecides can damage the liner.

Be sure to read the label before using any chemical in your pool.

Shock the pool

Shock treatment kills algae and other microbes in your water, so they don’t have time to grow and cause problems like stains or odors.

The best shock treatment also removes any organic materials such as dirt, debris, oils, bacteria, and dead cells from the surface of your pool water.

How to do it:

  1. Mix shock treatment with water according to package directions, usually one pound for every 10,000 gallons of pool water.
  2. Shake the mixture well before adding it to your pool.
  3. Pour or broadcast the shock evenly across the surface of the pool using a clean plastic container with a pouring spout or a clean plate, being careful not to splash the mixture into your pool filter.
  4. For a vinyl-liner pool, you can use a plastic baggie with a corner cut off instead of using a plate.
  5. Let the solution circulate for 24 hours while agitating it occasionally by running your pool pump and filter.

Please note that all shock treatments contain cyanuric acid (stabilizer), which can damage your pool liner if not removed before backwashing the filter or refilling your pool.

Run the pump & filter 24/7

pool circulation pump and filter

Give algae no place to grow by keeping your pool water circulating constantly.

How to do it:

  • Run your pool pump and filter 24 hours a day.

For best results, adjust the filtration system, so the water flow rate is approximately equal to the amount of pool water you’re replacing every hour.

If you have an automatic cleaner, run it while keeping your pool pump running.

Flock the pool

Pool flocking is the process of adding a special chemical to your pool that will help to bind the algae to the walls and bottom of your pool. 

pool flocking

This will make removing the algae easier and prevent it from coming back.

Flocking is a process that involves introducing small pieces of polypropylene to the pool. The flocking material binds to the debris – or algae – because it has tiny barbs on one side.

The floc you pour into your pool attracts all sorts of stuff, making it clump together and sink to the bottom. From there, we can use filters or vacuums for more efficient cleaning!

Brush the sides and bottom of the pool

After flocking, use a brush or algae-covered vacuum head, which attaches to your telescopic pole, to remove algae from the pool walls and floor.

The algae should start to come off the walls and bottom of your pool, but you will still need to add some extra chlorine to ensure that all the dead algae are gone.

What causes algae stains in a swimming pool?

Many things can cause an algae outbreak, but the most common are high nitrogen or phosphorous levels, poor water clarity, exposure to sunlight, and stagnant water

Algae require three things to grow: sunlight, warm temperatures, and nutrients. If your pool has any of these things, you may have algae growing in it.

Green algae stains in pool

One type of algae you may find is green algae, which often appears as flat green spots on the walls and bottom of your swimming pool.

This type of algae is typically harmless but can cause discoloration if left untreated for too long. 

What causes green algae stains?

Green algae usually thrive when there are high levels of nitrogen or phosphorous in the water, poor water clarity (such as due to sun exposure), stagnant water conditions, or warm temperatures (over 78 degrees). On hot days, algae multiply rapidly.

green algae in outdoor pool

If you find that your pool is green, it’s best to treat the problem right away with an algaecide before putting any chlorine in the water because the 

Chlorine may not kill algae if it’s already colonized on the walls or floor of your pool.

Black algae stains in pool

It’s not good when you find green algae in your pool, but black algae are even worse.

No one should swim in your pool until it’s gone. The good news is that it is possible to kill black algae.

What is black algae?

Black Algae is a bacteria called cyanobacteria. It’s also called blue-green algae, but these little guys can appear black!

Black algae have a similar appearance to mold. So if you detect a lot of tiny black dots or large clusters of mold on your pool surfaces, you may have black algae.

How does black algae get in your pool?

black algae in river or lake

Usually, black algae grow in their pool because it comes from an outside source, a river or lake. 

It’s likely that a swimmer didn’t wash their swimwear and brought that stuff inside!

Is black algae harmful?

Yes, it actually can be harmful. Black algae can make anyone who uses your pool sick, and it gets even more severe if you accidentally drink infected water. 

Even the CDC suggests avoiding black algae at all costs.

That’s why it’s vital to identify black algae as soon as you see it and take immediate action to get rid of it.

How to identify black algae in your pool?

  • Your pool’s surface has black or blue-green dots and clusters
  • It thrives in porous grounds such as concrete, plaster, and gunite pools.
  • It is not as easy to brush off the wall with a regular pool brush or even one made of metal.
  • Fourth, it’s easy to remove the residue from your pool with some elbow grease.
  • It shows up even if your pool water is balanced, sanitized, and filtered because it enters your pool from an external water source.

How do you kill black algae?

Treating black algae isn’t the same as treating green algae. It takes a lot more effort and persistence. But it is possible to kill.

Okay, let’s get cleaning

  1. First things first- clean the filter. If you have black algae in your pool, you have black algae in your filter.
  2. Use test kits to ensure your water is balanced
  3. Brush the pool like there’s no tomorrow! Brushing black algae loosens the dirt from surfaces where Chlorine can kill them.
  4. Shock the pool repeatedly (up to 4 times)
  5. Run the pump for 24 hours. This will clear any excess shock and kill the algae in one swoop!
  6. You should brush the surfaces three to four times while chlorine levels are at an all-time high. This will get any remaining black algae off of walls and floor
  7. Clean the filter again
  8. Shock the pool again if necessary.
  9. Keep brushing, running the pump, and repeating steps if necessary.

How do you prevent black algae in the future?

The first line of defense is always to wash your swimsuits, floats, shoes, toys, and anything that you’ve may have used in a natural body of water before it goes back into your pool.

Wash using a washing machine and scrub all pool toys with proper sanitation.

Be sure to keep your pool properly balanced and sanitized to prevent any future algae growth. That means keeping your alkalinity, pH, and sanitizer levels in the recommended ranges at all times.

Run your pump and filter at least 8 to 12 hours a day all season long. Regularly vacuum and brush your pool. Shock your pool at least every week.

Keep your pool equipment clean, including skimmers, hoses, ladders, steps, the diving board, slides, solar blankets, and your safety cover. And make sure anyone using your pool rinses off first.

Yellow algae stains in pool

Mustard Algae is a curious form of yellow algae that only occurs in pools. It comes from outside sources, and it’s not something that can grow naturally within your swimming area either!

You might be surprised to learn that this odd algae forms after you’ve been swimming in rivers, lakes, or at the beach.

You’ll need to wash all clothes immediately because these pesky things stick well even after chlorine treatments.

Wash your bathing suits and pool toys

It’s important to wash all of the equipment that comes into contact with water. 

The first step in this process is washing clothes like suits or towels so they do not bring any pre-existing infection.

You can use a Clorox multipurpose spray or create your own.

To prevent mustard algae from coming back, mix ten parts of water with 1 part bleach. 

You can put this in a spray bottle and use it to clean your equipment, pool, toys, and floats.

Yellow algae is a pool nuisance that likes to cling and stick. You’ll need to brush it off or get into suspension.

Then you can follow these steps:

  • Make sure pH and alkalinity are in balance
  • Keep the chlorine high (5 parts per million)
  • Tripple shock the pool (at night time)
  • Let the pump and filter run overnight as you’re shocking the pool