Why Does My Pool Keep Getting Algae—And How to Get Rid of It

Why does your pool keep turning green—even though you clean it regularly?
And how are you supposed to stop it when it seems to grow back overnight?

If you’re tired of constantly fighting algae in your pool, you’re not alone. The number one reason homeowners struggle with algae is that the water isn’t properly balanced, even if it looks clean. In most cases, the chlorine level drops too low, or the pool’s filtration isn’t working hard enough to stop algae before it spreads.

In this article, you’ll learn exactly what algae is, what causes it to grow, how to kill it fast, and what professionals do to keep it from coming back.

What Is Algae and Why It Shows Up in Pools

Algae are tiny, plant-like organisms that grow quickly in warm, sunny water.
Even if you can’t see them yet, algae spores are always floating around your pool, as they are microscopic organisms. Once they find the right conditions—especially when chlorine is low, or water isn't properly balanced—they start to multiply rapidly.

Most homeowners are surprised to learn that algae problems are usually caused by simple water chemistry issues, like:

• Low chlorine levels

• Poor water circulation

• Warm temperatures and direct sunlight

• Organic contaminants like sunscreen, dirt, or leaves

• Tap water additives

What Causes Algae in Swimming Pools?

Before diving into common environmental and maintenance-related causes, it’s essential to understand that algae problems can start even with the water you fill your pool with.

Tap Water Additives and Algae Growth

Municipal tap water often contains additives such as silicates and nitrates, introduced by city water treatment plants to prevent pipe corrosion and maintain the safety of drinking water per CDC guidelines. While helpful in plumbing systems, these compounds can unintentionally feed algae in pools. Nitrates, in particular, are a key nutrient that helps algae reproduce. Because these additives are dissolved and invisible, they’re also difficult to remove using standard pool filtration. This is one reason algae outbreaks are becoming more frequent, even in freshly filled or well-maintained pools.

In rural areas where well water is used to fill pools, this and other factors should be considered, as many different contaminants in water may be the source of staining and may also affect the traditional water balancing steps.

Now, let’s break down the most common causes of algae that every pool owner should watch out for:

1. Low Chlorine Levels

Chlorine is your pool’s primary defense against algae—but it doesn’t take much for levels to drop.
Hot weather, heavy swimmer loads, and even rain can quickly burn through chlorine, giving algae a chance to grow.

2. Poor Filtration and Circulation

Still, water is where algae thrive.
If your pool pump isn’t running long enough each day (6–8 hours minimum in summer) or there are “dead spots” where water doesn’t flow well, algae can start to build up.

3. Swimmers and Pool Toys

Algae spores can hitch a ride on anything that’s been in a lake, river, or ocean—like swimsuits, floats, or life jackets.
After a pool party, if you don’t shock and clean the pool properly, algae can appear fast.

4. Yard Fertilizers or Runoff

Many fertilizers contain nitrogen and phosphorus—key food sources for algae.
If you’re treating your lawn and it rains or the sprinklers overspray, that runoff can end up in your pool.

5. Summer Heat, Pollen, and UV Exposure

In areas like Los Angeles, extreme heat and pollen blooms are major algae triggers.
Temperatures that regularly exceed 100°F speed up algae growth and burn through chlorine faster than your system can keep up. Combine that with spring and summer pollen floating into the pool, and algae spores have a perfect environment to spread.

6. Summer Storms

Though less frequent in LA, when summer storms do roll in, they can drop debris, flood water, and nitrogen into your pool—giving algae everything it needs to bloom.

Pro Tip:  Consult with a professional to determine the TDH (amount of water circulating through your system) and the turnaround time your pool's water requires.

Common Types of Pool Algae

Green Algae

The most common type. Grows fast, turns the pool green, and floats in the water or clings to walls.

Mustard/Yellow Algae

Tends to grow in shady areas. It’s harder to kill and often comes back if not eliminated.

Black Algae

Actually, a type of bacteria. Grows in dark spots and forms a protective layer—very hard to remove without brushing and special treatment.

Pink Slime / White Water Mold

It's not algae, but it can be mistaken for it. Pink slime is a bacteria that sticks to surfaces; white mold looks like floating tissue paper.

How to Kill Algae in Your Pool,  Step-by-Step

1. Shock the pool with liquid chlorine.
   Raise chlorine to 10–20 ppm to break through the algae’s defenses.

2. Brush the pool thoroughly
   Loosen algae from surfaces so chlorine can reach it.

3. Add a clarifier
   A natural chitosan-based clarifier helps clump dead algae, allowing your filter to catch it.

4. Run your filter 24 hours a day until the water clears

5. Clean the filter afterward (As often as needed)
   Algae can get stuck inside and recirculate if the filter isn’t cleaned properly.

6. Test for phosphates and treat if needed
   Phosphates feed algae. Keeping them under 500 ppm helps prevent regrowth.

How to Prevent Algae for Good

Keep Chlorine Levels Consistent

Check and adjust the chlorine level daily in the summer. A target FC: CYA ratio of 5% and not exceeding 7% keeps sanitizer levels strong.

Brush and Vacuum Weekly

Brushing removes biofilms that protect algae. Focus on corners and shaded areas.

Run the Filter Long Enough

6–8 hours per day minimum. More during hot weather or after a pool party.

Shock After Heavy Use

After big swim days or storms, use a shock treatment to reset your chlorine levels.

Control Phosphates

Use a phosphate remover regularly to reduce algae’s food supply.

Now you know why algae keep coming back—and what steps you can take to stop it for good.

Whether it’s low chlorine, poor circulation, or leftover nutrients, the key is to control the conditions algae needs to grow.
Want a stress-free pool this summer? Schedule a water test or ask us about our algae-prevention service plan.

At J Designs, we help pool owners enjoy clean, clear water with less guesswork—and no green surprises.

 Tips for an Algae-Free Pool is a popular guide for homeowners to combat algae.

“Still unsure if your pool chemistry is working against you? Read our Keeping your water balanced.”

“Say goodbye to green pool water. Book a free pool health inspection with J Designs today!”