Can Algae Stain a Gunite Pool? Expert Answers Inside
- Adam Allen
- 3 hours ago
- 7 min read
Green streaks, cloudy walls, stubborn stains that won’t brush off? You might be wondering, can algae stain a gunite pool? Yes, it can—and if it’s left too long, those stains sink into the surface, not just sit on top.
At Ferrari Pools: Custom Outdoor Living, we’ve worked with gunite pools for decades. We’ve seen how fast algae can leave a mark, especially when the water’s warm and circulation isn’t great.
In this blog, we’ll cover why algae stains happen, how to tell what you’re dealing with, how to clean it, and how to keep your pool looking its best.
Key Takeaways
Algae can stain a gunite pool if it’s left untreated, but the right cleaning routine keeps your pool surface spotless.
Green stains on pool plaster usually mean it’s time to rebalance your pool water and step up brushing and filtration.
Staying consistent with maintenance is the easiest way to keep your pool clean and avoid costly stain removal later.
What Is Algae and Why It’s a Problem for Gunite Pools
Algae are organic microorganisms that thrive in warm, still pool water, especially when chemical levels are off. If your filtration system isn’t running properly, algae growth can take over quickly.
Gunite pools are more vulnerable because their porous surface holds onto organic stains and debris. Unlike fiberglass pools, gunite has a rough texture that lets algae settle into tiny pockets. Poor circulation, shady corners, and high water levels all create ideal spots for staining.
Common types that leave marks include:
Green algae: The most common form. It’s fast-growing and often leaves behind green stains on pool walls or the pool floor, even after the algae itself is gone.
Black algae: Deeply rooted and tough to remove. It clings to rough plaster and resists basic treatment.
Mustard algae: Yellowish or light brown in color. It hides in low-flow areas and is more chlorine-resistant than green algae.
Algae can be killed, but stains often remain on the pool surface unless properly treated.
What Do Algae Stains Look Like?
Algae stains usually show up as green stains, brownish-green patches, or even blue-green discoloration across the pool surface. You’ll often find them clinging to shady, low-flow areas like:
Steps or benches where the pool water circulation is weak
Corners or recessed areas
A band of green staining along the pool walls at the water level
If brushing with stainless steel bristles and liquid chlorine doesn’t remove the mark, you're likely dealing with a stained area, not live algae. Here’s how to tell:
Green stains on pool plaster or floors that won’t scrub off
Spots that show up after an algae bloom but stick around
Fading happens slowly, if at all, without targeted treatments like muriatic acid, granular chlorine, or ascorbic acid
Not all stains are from algae. Metal stains, like copper or iron, can look greenish-blue or rust-colored and are often mistaken for algae. If the stain doesn’t respond to chlorine, testing your chemical levels and water source can help confirm what’s causing the issue.
If you’re unsure, bring in a pro to test the staining and determine the right treatment for your pool.
What Causes Algae Stains in Gunite Pools?
Several common issues can lead to green stains on pool plaster or floors, especially in gunite pools, where the porous surface makes it easier for algae and organic matter to settle in.
Low chlorine levels give algae a chance to grow fast and deep
Poor circulation in corners, steps, or dead zones traps debris
Warm temperatures and sunny weather speed up algae growth
High water levels or still water reduce flow and filtration effectiveness
Skipping regular brushing lets dirt, pollen, and leaves build up on the pool surface
Filtration systems that aren’t run long enough allow spores to linger
Organic debris left in the water feeds growth and causes organic stains
Once these conditions line up, algae can leave behind tough stains that even chlorine won’t remove. Keeping your pool clean with consistent care is key to preventing green staining that sinks into the plaster.
How to Remove Algae Stains from a Gunite Pool
Once algae leaves its mark, regular cleaning won’t always cut it. To get your pool surface looking clean again, you'll need a mix of manual scrubbing, the right chemicals, and possibly some specialized equipment. Here's how to handle those stubborn stains for good.
Manual cleaning methods
Start by brushing the surface using a brush with stainless steel bristles (especially on plaster or rough gunite) to remove surface buildup. Vacuum the bottom thoroughly. Clean out the skimmers and filters so debris and algae don’t get recirculated.
Chemical treatments
If brushing isn’t enough, chemical options can help lift green stains and algae residue from the pool plaster. Use the right treatment based on the stain type and surface.
Granular chlorine shock: Raise chlorine quickly to kill off residual organic matter.
Muriatic acid spot treatment: Use with extreme care on plaster surfaces to remove deep stains.
Ascorbic acid: A gentler option for some stained surfaces or for metal‑related discoloration.
Specialized stain kits: Some treatments target both algae‑based stains and metal/mineral stains.
Specialized equipment
Power washing can help remove embedded stains from rough gunite, but only when done cautiously so you don’t damage the surface. Robotic cleaners or automatic vacuums help maintain the surface after major treatments.
How to Prevent Algae Stains in a Gunite Pool
Prevention is far cheaper than full stain removal:
Maintain chlorine in the acceptable range (1.0‑3.0 ppm) and pH between 7.4 and 7.6.
Brush walls and floors weekly, or more often in warm months.
Run your filtration system 8–12 hours daily so water keeps moving and doesn’t stagnate.
Use an algaecide during warm weather to stop algae before it starts.
Remove leaves and organic debris promptly. Stay on top of the water level so skimmers and flow continue properly.
In porous surfaces like gunite, you can’t let things sit. Those pores will trap organic stains fast.
When It’s NOT Algae: Common Stain Lookalikes
Not every stain means algae. Consider these possibilities:
Metal stains (iron, copper, manganese): often blue‑green or rust colored.
Calcium scale: white, crusty buildup on plaster.
Organic stains: from leaves or bugs that sat too long.
Tests help identify the cause: a vitamin C (ascorbic acid) tablet over the stain may cause a reaction for metal stains. If no reaction, it is likely organic or algae‑related. If identification is unclear, call a pro. Misdiagnosing costs time, money, and may damage the surface.
Is It Worth Draining or Acid Washing the Pool?
Draining or full acid washing are major steps, and often last resorts. You should only consider them if:
The stain won’t respond even after brushing, chemical treatment, and stain kits.
The pool is being resurfaced or is already scheduled for full repair.
Risks of acid washing: It removes a thin layer of plaster with each treatment, shortening your surface’s lifespan. Instead, try spot acid washes, chlorination, and stain kits first. Save full drain/acid for when the pool’s condition justifies it.
How Often Should You Brush and Maintain a Gunite Pool?
Consistency beats one‑time fixes. For our Massachusetts conditions:
Brush walls/floor daily during peak warm months, or at least 2‑3 times each week.
Test chemical levels weekly.
Clean filters monthly.
Inspect, shock, and check corners, steps, and flow zones seasonally.
In our region, weather and leaf‑fall vary a lot, so staying ahead means fewer stains, clearer pool water, and fewer surprises.
Can You Avoid Algae Stains Altogether?
Here’s the honest truth: avoiding every stain may not be realistic with a gunite finish. The good news is: with a strong routine, you can keep them rare. Smoother finishes (tile, aggregate) resist stains better, but if you stick to a maintenance plan, you’ll prevent most major issues.
Sick of Pool Stains That Keep Coming Back?
If you’ve tried brushing, shocking, and balancing your pool water but the stains still won’t go away, you’re not the only one. Green stains, marks on pool plaster, and stubborn discoloration can be tough to deal with, especially in gunite pools where stains sink into the surface.
That’s where we come in.
At Ferrari Pools: Custom Outdoor Living, we’ve spent over 65 years helping pool owners fix stains and keep their pools clean. We handle everything from algae removal and chemical balancing to resurfacing and full maintenance.
Want a cleaner pool without all the guesswork? Call Ferrari Pools: Custom Outdoor Living for a free on-site consultation.
Conclusion
Yes, algae can stain a gunite pool. These marks go beyond surface level and can settle deep into the plaster. But they don’t have to be permanent.
With the right cleaning techniques, good chemical balance, and consistent upkeep, you can clear existing stains and prevent new ones from forming. At Ferrari Pools: Custom Outdoor Living, we’re here to help you keep your pool clean and looking great all season long.
Frequently Asked Questions
Can algae permanently stain a pool?
Yes, algae can leave permanent stains, especially in gunite pools. The porous surface absorbs stains, so even after the algae are gone, green stains on pool plaster can stick around. If brushing and chlorine don’t work, try ascorbic acid, a stain kit, or call in a pro before it sets in for good.
How to get algae off gunite pool?
Brush with a stainless steel brush
Vacuum and remove debris
Shock with granular chlorine
Treat stubborn stains with muriatic acid or ascorbic acid
Run your filtration system daily to prevent regrowth
Still seeing stains? It may be time for a deeper clean or professional help.
Why does my pool water look clear but still have green stains on the pool walls?
Even if your pool water looks clean, green stains on pool walls can stick around after algae is gone. These are organic stains left behind and often need targeted treatment like muriatic acid or granular chlorine to fully remove.
How do I know if it's algae or metal causing green stains on my pool surface?
Test the area with chlorine and a vitamin C (ascorbic acid) tablet. If granular chlorine makes it fade, it’s likely algae. If ascorbic acid works, it may be a metal stain. Either way, adjusting chemical levels and checking your filtration system is a good move.
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