Green pool water is one of the most common problems Charleston pool owners face. The heat, the humidity, the afternoon thunderstorms that dump nutrients into the water. It all adds up fast. And once algae takes hold, it can spread across your entire pool in a matter of days.

The good news? It’s fixable. And with the right habits, it’s preventable.

In this guide, we’ll walk you through what causes pool algae, how to identify the type you’re dealing with, how to clear green pool water step by step, and how to stop it from coming back.

 

What is Pool Algae?

Pool algae is a microscopic plant organism that thrives in warm, stagnant water. When conditions line up (and in Charleston, they line up often), algae can multiply rapidly and turn your pool water green, cloudy, or worse.

Algae isn’t just ugly. It makes surfaces slippery, clogs your filtration system, and creates an environment where harmful bacteria can grow. Swimming in a pool with visible algae is not safe for you, your family, or your pool equipment.

 

The Three Types of Pool Algae

Not all algae look or behave the same. Knowing which type you’re dealing with helps you treat it faster and more effectively.

Green Algae

Green algae is the most common type found in residential pools. It shows up as a slimy film on the water surface, walls, steps, and ladders. It spreads quickly, especially in warm weather with direct sunlight. The upside: green algae is the easiest type to treat.

Black Algae

Black algae is the stubborn one. It appears as dark spots or patches on pool surfaces, usually on concrete or plaster walls. Black algae develops a protective outer layer that makes it resistant to normal chlorine levels. It roots deep into porous surfaces, which means brushing alone won’t get rid of it. Black algae requires aggressive treatment and often professional help.

Yellow (Mustard) Algae

Yellow algae is less common but harder to deal with than green. It shows up as gold or yellowish patches on walls and floors, usually in shaded areas that get less circulation. Mustard algae is resistant to standard chlorine levels, so it often comes back after treatment if you don’t address it thoroughly.

 

Why Does Algae Form in Your Pool?

Algae needs a few things to take hold: warmth, sunlight, nutrients, and time. In Charleston’s climate, the first two are a given from April through October.

Here’s what typically triggers an algae bloom:

Unbalanced water chemistry. When your pH drifts too high (above 7.8), chlorine becomes less effective. Algae thrives in alkaline environments, so even a small shift can open the door.

Low chlorine levels. Chlorine is your primary defense against algae. When free chlorine drops below 1 ppm, algae has room to grow. This happens faster than you’d expect after a rainstorm, a pool party, or a few days of intense sun.

Poor circulation. Stagnant water gives algae time to settle and spread. Dead spots in your pool where water doesn’t move, like behind ladders, in corners, or along the waterline, are where algae shows up first.

Dirty or clogged filters. If your filter can’t remove debris and organic matter from the water, those particles become food for algae.

High phosphate levels. This is one most homeowners overlook. Phosphates are the primary food source for algae. If your phosphate level is elevated, no amount of shock and algaecide will solve the problem long-term. You have to reduce the phosphates first.

 

Can You Swim in a Pool with Algae?

No. If you can see algae in your pool, stay out.

Algae itself can cause skin irritation and rashes. But the bigger risk is what grows alongside it. Algae-contaminated water often harbours harmful bacteria like E. coli that can cause ear infections, gastrointestinal illness, and other health issues. It also makes pool surfaces dangerously slippery.

Don’t swim until the water is fully treated, tested, and clear.

 

How to Fix a Green Pool: Step by Step

If your pool has already turned green, here’s how to bring it back. This process works for green algae. For black or yellow algae, you may need professional treatment.

Step 1: Test the Water

Start by testing your water chemistry. Use a reliable test kit or take a sample to your local pool supply store. Check your pH, free chlorine, alkalinity, and phosphate levels.

You’re looking for:

• pH: 7.2 to 7.8 (ideally 7.4 to 7.6 for shock treatment)
• Free chlorine: you’ll be raising this significantly in the next step
• Phosphates: if elevated, treat with a phosphate remover before shocking

Step 2: Brush Everything

Before you shock, grab a stiff pool brush and scrub the walls, floor, steps, ladders, and corners. This breaks up the algae’s hold on surfaces and exposes it to the water, where the chlorine can actually reach it.

Pay extra attention to shaded areas, behind ladders, and along the waterline. If you have plaster or concrete surfaces, use a steel-bristle brush. For fiberglass or vinyl, use a nylon brush to avoid surface damage.

Step 3: Shock the Pool

Shock your pool with a heavy dose of calcium hypochlorite (cal-hypo) or liquid chlorine. For a green pool, you need to raise the free chlorine level to at least 10 ppm. A severely green pool may need 20 to 30 ppm.

Follow the dosing instructions on the product label based on your pool’s volume. Shock in the evening so the chlorine has overnight to work without UV degradation from the sun.

Step 4: Run the Filter Continuously

After shocking, run your pump and filter 24 hours a day until the water clears. This is not the time to run your pump on a timer. The filter needs to circulate the treated water continuously to remove dead algae from the pool.

Clean or backwash your filter every 8 to 12 hours during this process. The filter will clog faster than normal because it’s pulling out a large volume of dead algae. If you skip this step, the filter loses efficiency and the pool stays cloudy.

Step 5: Vacuum the Dead Algae

As the water starts to clear, you’ll see dead algae settling on the bottom. Vacuum it out. If the bloom was heavy, vacuum to waste (bypassing the filter) to avoid pushing all that debris back through your system.

Step 6: Test Again and Balance

After 24 to 48 hours, test the water again. You want to confirm:

• Free chlorine is holding above 1 ppm (if it dropped to zero, the algae consumed it and you need to shock again)
• pH is between 7.2 and 7.8
• Alkalinity is between 80 and 120 ppm

If the chlorine held and the water is clearing, you’re on the right track. If the water is still green or the chlorine dropped to zero overnight, repeat the shock treatment. Some blooms take two or three rounds.

Step 7: Add Algaecide as a Follow-Up

Once the water is clear and the chlorine level has come back down to normal operating range (1 to 3 ppm), add a maintenance dose of algaecide as a preventive layer. This is not a replacement for proper chlorine levels. It’s a backup that makes it harder for algae to re-establish.

 

How to Prevent Algae from Coming Back

Getting rid of algae is only half the job. Keeping it away is what matters.

Maintain balanced water chemistry. Test your water at least once a week. Keep pH between 7.2 and 7.8, free chlorine between 1 and 3 ppm, and alkalinity between 80 and 120 ppm. In Charleston’s summer heat, test twice a week. Chemistry shifts faster than you’d expect.

Run your pump long enough. Your pool water should turn over at least once every 8 to 12 hours. In the summer, running the pump 10 to 12 hours a day is the minimum. Poor circulation is one of the top causes of algae growth.

Brush and skim regularly. Skim the surface daily to remove leaves, pollen, and debris before they sink and decompose. Brush walls and floors weekly to prevent algae from getting a foothold on surfaces.

Shock your pool weekly. A weekly shock treatment during the swimming season keeps chlorine levels high enough to kill any algae spores before they bloom. This is especially important after rainstorms, heavy pool use, or extended periods of high heat.

Clean your filter on schedule. A dirty filter can’t remove the particles that feed algae. Backwash or clean your filter cartridge on the manufacturer’s recommended schedule, and more frequently during peak season.

Manage phosphate levels. If you’re doing everything right and algae keeps coming back, test your phosphates. Elevated phosphate levels are often the hidden culprit. Use a phosphate remover to bring the level down, and the algae problem usually resolves.

Use a pool cover when you can. A pool cover reduces debris, limits UV exposure, and slows evaporation. All three of those factors contribute to algae-friendly conditions.

 

FAQs

 

When DIY Isn’t Enough, Heritage Pools Can Help

Some algae problems are straightforward. Test, shock, brush, filter, done. Others are stubborn, recurring, or deep enough that they need professional equipment and experience.

Heritage Pools offers a green-to-clean service designed for exactly this situation. Our team assesses the type of algae, tests your water chemistry, treats the pool, and gets it back to clear. We also identify the root cause, whether it’s phosphates, a circulation issue, or equipment that isn’t performing, so the problem doesn’t come back next month.

If you’re a Charleston-area homeowner dealing with a green pool that won’t cooperate, give us a call. We’ll handle it from here.

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