What Causes Algae In A Pool? Causes, Treatment Options, And Prevention Tips

Nothing more awful and discouraging than preparing for a swim and getting welcomed by algae. Perhaps you have been in this battle for months and it doesn’t seem like they are leaving anytime soon. First, you should find out the root causes of seeing algae in your pool.

The reason behind it is just as important as the solution you plan to apply. Algal growth spreads very fast; the earlier you trace the causes, the better for your pool time.

Read on to find out what causes pool algae, why they won’t leave, and how to get rid of them.

What causes algae in the pool?

Algae could be in your pool for the following reasons:

1. Contamination

Algae spores can enter the pool through the wind, rain, swimsuits, or contaminated cleaning tools. They are initially present in minute amounts and microscopic sizes.

Gradually, an algae bloom occurs when chlorine level dips, pH and temperature rise, or when there is an imbalance of nitrates, phosphates, and carbon dioxide.

Furthermore, algae are photosynthetic organisms. They carry out photosynthesis to feed and grow, expanding across the pool. Contaminants like dust specks, fallen leaves, and the remains of previous algae also provide food for existing algae spores, encouraging their growth and spread.

2. Improper filtration

Improper filtration and water circulation also support algae bloom. A blocked filter encourages bacterial growth, and algal growth consequently. These algae clog the filter pores and make it of no effect.

Algae growth spreads into crevices, over the pool tiles and walls, behind the pool lights, and under the ladder and vinyl pool liners. Some of them grow roots deep into the plaster and create unappealing stains.

Types of pool algae

Green algae

These are the types of algae everyone is familiar with. They are green, as their name implies, and turn pool water hazy. Green algae are floating flat organisms found on the surface and walls of the pool. They also appear in rough areas and in places where water circulation is low.

The green algae spread quickly forming a slimy, slick film. However, they are the easiest to get rid of.

Black algae

Black algae are the blue-green algae. These are the most stubborn types of algae. They have strong roots that penetrate the pool plaster.

They also have a protective layer that protects them from the destroying effect of chemicals. It would be best if you destroyed their roots to get rid of them completely.

These algae appear as black or blue-green spots in different areas of the pool. They can be as small as the size of a pencil eraser tip or as big as a quarter. Black algae are so stubborn and invasive that they can bloom even when in proper sanitization and filtration.

Unlike other types of algae, the black algae contamination could be from contaminated pool cleaning equipment or from the swimsuit of a person who has been around ocean water.

Yellow algae

Yellow algae or mustard algae appear as flat sheets like the green algae but are difficult to get rid of, like the black algae. They are chlorine-resistant and highly resilient. They can survive algae treatment and cause a re-infection. However, they are quite rare.

Problems pool algae cause

  • Algae in the pool colors the water and makes it uncomfortable
  • Getting rid of algae can be time-consuming and expensive
  • As algae release carbon dioxide into the water, it raises the pH level
  • One algae bloom will always lead to another

How can you get rid of algae in the pool?

Wash the pool

If you think back and realize it’s been a long time since you washed the pool, a good scrub can help get rid of algae if they are still in the early stages of growth. Use strong cleaning agents such as bleaches to scrub off the algae sheets.

If the water is cloudy, use a flocculant to clear all particles. Wash the drain and make sure all debris is flushed through the filtration system.

Alter the pH of the water

Algae thrive at extreme pH levels. If you lower the pH of the pool, you will be making their environment uncomfortable for them, and they will be gone in almost no time. Keep the pool’s pH to 7.0 and not higher than 7.4.

Drain the pool

You may fully or partially drain the pool to eliminate debris and murky water. If you drain it fully, wash and refill it. If you drain halfway, remove as much debris as you can while you drain. Afterward, treat with an algaecide and brush the pool.

You can use a clarifier (if necessary). Then, run the pool filter for at least 24 hours until you have a clear pool.

Shock the pool

After altering the pH of the pool, give the pool a good shock with unstabilized chlorine. Take the chlorine level to 30 ppm, where it becomes inhabitable for algae. Pump water for at least 24 hours until the pool is saturated with chlorinated water.

You may wonder if you can bathe in this water. Not to worry, after good exposure to sunlight, the chlorine level will go down and become tolerable for you.

Get a saltwater chlorinator

This is quite an investment but if you run a pool for commercial purposes, it would be worth it. A saltwater chlorinator helps to keep chlorine levels constant and gives algae almost no opportunity to bloom.

How to prevent future algae growth in the pool

  • Maintain a high pH and low chlorine levels
  • Wash and sanitize your pool regularly
  • When cleaning your pool, use a brush to scrub the tiles to remove lodged dirt from the pores of the plaster
  • Employ stabilizers like cyanuric acid to protect the sun from lowering chlorine activity, thereby hindering algae bloom
  • Use filter cleaners to clean the filter and eject organic matter and other contaminants that serve as algae food and encourage bloom

Can you swim in the pool with algae?

Swimming in a pool with algae is not advisable. Algae in the pool encourages the growth of bacteria that feed on algae. These bacteria can cause skin rash, skin irritation, nausea, stomach aches, and a tingling sensation on the fingers and toes.


At what pH does fungi thrive in pools?

Algae grow well in environments with high pH, 8.2 – 8.7. This is why lowering pH is an effective way to get rid of algae in your pool.

Can baking soda get rid of algae in the pool?

Baking soda can help remove algae from the pool not destroy them. Scrubbing areas with algal growth can help get rid of the green patches on the walls of your pool. Moreover, adding baking soda to pool water can raise the pH.

Why is the pool green after removing algae?

If your pool water is still green after removing algae, it could be a sign of too high pH, a spike in copper levels because of the algaecide, or the presence of pollen and chlorine-resistant algae.

The green hue could also be due to the presence of metals and phosphates in the water, and an extremely high cyanuric acid level.


When next you see algae in your pool, trace the cause before trying out every solution you know. Check the pH of the water, the filter, the pipes, and the last sanitization date. Remember that your pool needs cleaning, just like your bathroom and bedrooms.

Keep pH levels in check, put a daily filtration system in place, clean your pool regularly, and go chemical on them with an algaecide.

A quick run over the standards: pool’s pH should be at 7.2, alkalinity should be 100 ppm, chlorine level is best between 2 and 4 ppm, and cyanuric acid level should be between 30 and 50. Here’s goodbye to murky water!

Here’s a comparison of fungi vs algae to help you differentiate them correctly.

Thanks for reading.