How to Remove Algae Stains from Pool Surfaces

How to Remove Algae Stains from Pool Surfaces

Algae stains are one of the most common pool maintenance headaches. These green, yellow, or even black marks cling to your pool’s walls and floor and can stick around even after shocking the water. The stains are not just ugly. If left untreated, they can damage your pool surface and become harder to remove over time.

The good news is you can remove algae stains from your pool without draining it or using harsh chemicals. All it takes is the right tools, a little patience, and a few simple steps.

What Causes Algae Stains in Pools?

Algae stains come from algae blooms that attach to your pool’s surface. Even after killing the algae in the water, the pigments and residue can remain stuck to walls, steps, and the pool floor.

Here are some of the main reasons algae stains appear:

• Poor water circulation
  
• Unbalanced chlorine or sanitizer levels
  
• High phosphate levels (algae food)
  
• Warm, sunny conditions
  
• Infrequent brushing or vacuuming
  
• Algae spores introduced by swimmers or wind
  

Not all algae stains are the same. Knowing what type you’re dealing with helps you treat it properly.

Types of Algae Stains and What They Look Like

There are three common types of algae that can cause stains in your pool:

1. Green Algae
   
   • Most common type
     
   • Leaves slimy green stains on steps and walls
     
   • Often caused by low chlorine levels or poor circulation
     
2. Yellow (Mustard) Algae
   
   • Light brown or yellowish stains
     
   • Usually found in shady areas of the pool
     
   • More resistant to chlorine than green algae
     
3. Black Algae
   
   • Dark black or blue-green spots
     
   • Appears in cracks or porous surfaces like plaster
     
   • Has deep roots that require aggressive treatment
     

Tools and Supplies You’ll Need

Before you start, gather the right tools for the job:

• Stiff pool brush (nylon or stainless steel depending on surface type)
  
• Algaecide (specific to the type of algae)
  
• Chlorine-based pool shock
  
• Pool vacuum or automatic cleaner
  
• Pool test kit or strips
  
• Water clarifier (optional)
  
• Pumice stone (for plaster or concrete pools only)
  
• Safety gloves and goggles
  

Avoid using household cleaners not labeled for pool use. They can throw off your water chemistry or damage surfaces.

How to Remove Algae Stains from Pool Surfaces

1. Test and Balance Your Pool Water

Before you tackle the stains, make sure your water is properly balanced:

• pH: 7.2 to 7.6
  
• Alkalinity: 80 to 120 ppm
  
• Chlorine: 1 to 3 ppm
  

Use a reliable pool test kit and adjust as needed. Lower pH levels slightly (closer to 7.2) can make chlorine more effective when treating stains.

2. Brush the Affected Areas

Use a stiff brush to scrub the algae stains. Focus on problem areas:

• Steps and ladders
  
• Corners and behind lights
  
• Pool floor and walls
  

Use a circular motion to break up algae and expose any roots hiding in surface textures.

3. Shock the Pool

Next, shock the pool with a chlorine-based product:

• Use 1 to 2 pounds per 10,000 gallons of water
  
• Pour it around the perimeter
  
• Run the pump for at least 8 hours
  

For black algae, triple shock may be necessary. Always follow product directions closely.

4. Use an Algaecide

Once the shock has circulated, apply an algaecide specific to the algae type. For best results:

• Add it near stained areas
  
• Brush again after 30 minutes
  
• Let the pump run overnight
  

Never mix algaecide and shock at the same time. Wait until chlorine levels drop before adding algaecide.

5. Vacuum and Filter

After treatment:

• Vacuum the pool thoroughly
  
• Backwash or clean your filter
  
• Continue running the pump for at least 24 hours
  

This helps remove dead algae and clears up cloudy water.

6. Spot Treat Stubborn Stains

For stains that didn’t go away:

• Try a stain eraser or chlorine tablet held directly on the spot (wear gloves)
  
• Use a pumice stone for plaster surfaces (avoid on vinyl or fiberglass)
  
• Repeat brushing and shocking if needed
  

Extra Tips for Stain Prevention

Once your pool is clean, keep it that way by staying ahead of algae buildup:

• Test your water weekly
  
• Keep chlorine between 1 and 3 ppm
  
• Brush your pool walls and floor once a week
  
• Use phosphate removers to starve algae
  
• Run the pump for 8 to 12 hours daily during swim season
  
• Shock the pool weekly or after heavy use
  
• Clean filters and skimmers regularly
  

When to Call a Professional

If stains won’t go away or if you see black algae coming back over and over again, it might be time to call a pool technician. They can use advanced treatments like acid washing or surface refinishing, depending on the severity.

Professional help can also prevent damage to expensive surfaces or equipment.