If you’ve ever taken a close look at your swimming pool and noticed an uninviting brown buildup on the walls, you’re not alone. Many pool owners are baffled by this discoloration. It not only detracts from the beauty of the pool but also signals that something may be off in your water chemistry or maintenance routine. So, what exactly is the brown stuff on my pool walls? In this comprehensive article, we break down the most common causes, why it appears, and how to safely and effectively remove it.
Common Causes of Brown Discoloration on Pool Walls
The brown substance you’re seeing is not always the same thing. Depending on your pool type, water source, and maintenance habits, the cause could be any of the following:
1. Iron Staining (Most Common Culprit)
Iron in the water is one of the leading causes of brown staining on pool walls and floors. Iron may come from:
- Municipal or well water sources
- Metal pool equipment like pumps or filters
- Old pool pipes made of iron
When iron in the water is oxidized — most commonly through the introduction of chlorine — it turns into a rust-colored stain. These stains can appear orange, brown, or even black depending on the concentration and oxidation level.
2. Organic Debris Accumulation
Outdoor pools are especially vulnerable to leaves, dirt, pollen, and other vegetation. Left unattended, these materials can decompose and leave a brownish residue on your pool walls. This buildup usually appears in lower sections of the pool or on shaded areas where water circulation is minimal.
3. Algae Colonization
While algae typically appears green, black, or mustard-colored, certain algae strains can present as dark or brown-looking — especially mustard algae or black algae that have begun to die or break down. Algae growth is more common if:
- Pool chemistry is unbalanced
- Chlorine levels are low
- The pool isn’t brushed regularly
- There’s poor circulation or stagnant water
4. Mold or Mildew Buildup
In moist pool environments, particularly in enclosed or indoor pools without proper ventilation, mold or mildew can develop. These fungi often appear in corners, under the waterline, or near return jets. They can create dark or brownish patches that are sticky or powdery to the touch.
5. Chemical Imbalance
pH imbalance, high alkalinity, low sanitizer levels, and improper calcium hardness can all contribute to staining, scaling, and discoloration. For instance:
| Water Imbalance | Consequence |
|---|---|
| High pH | May lead to calcification — chalky deposits on walls |
| Low Chlorine | Promotes algae growth which appears as brownish stains |
| High Calcium Hardness | Accumulates as a white or brownish scale |
Identifying the Exact Problem: What Should You Do?
Before jumping to cleaning solutions, it’s essential to correctly identify what’s causing the brown material. Misdiagnosis may lead to wasted time or even damage to your pool surface. Here’s how you can distinguish between the main sources of brown discoloration:
Iron vs. Algae Check: The Chlorine Tablet Test
Step-by-step procedure:
- Place a chlorine tablet on the affected area of the pool wall.
- Observe what happens after 10–15 minutes.
| Result | Interpretation |
|---|---|
| Area bleached white or disappeared | You’re dealing with algae, organic debris, or a mild mold issue |
| No effect or worse discoloration | Metal stains — likely iron, so a chlorine-based cleaner won’t help, and may even oxidize further |
Water Testing: Your Best Defense
Before proceeding with stain removal, test your pool carefully using:
- A high-quality test kit (not just test strips)
- Test for pH, Total Alkalinity, Calcium Hardness, Cyanuric Acid, Free Chlorine
If imbalances are detected, you should correct those first before addressing the staining itself.
How to Clean and Remove Brown Stains Based on Cause
Once you know what you’re dealing with, you can deploy the appropriate strategy to eliminate the brown stuff from your pool walls.
1. Removing Iron Stains from Pool Surfaces
Metal staining — including those from iron — typically requires specialized chemistry.
Steps to Remove Iron Deposits:
- Remove chlorine completely using a chlorine neutralizer.
- Broadcast a metal stain remover like ascorbic acid or citric acid directly onto the stains.
- Use a soft pool brush (or fiber brush for concrete pools) to scrub the stains until they loosen.
- Run the filter and backwash as needed to remove dislodged particles.
If persistent, use a metal sequestrant or chelating agent to bind any remaining particles and remove them through filtration.
2. Clearing Organic Material Buildup
This includes dirt, leaves, algae, and pollen stains that look brown due to decomposition or limited exposure to chlorine.
Steps to Clear Organic Stains:
- Brush the affected area thoroughly — preferably with a nylon or stainless-steel brush.
- Shock the pool with a high dose of chlorine
- Run the pump for at least 12 hours following.
- Brush again, then vacuum to waste if applicable.
Adding a pool clarifier or flocculant can also help trap floating debris that’s contributing to cloudy or staining residue.
3. Addressing Algae Growth on Pool Walls
Depending on the type of algae you’re contending with — green, mustard, or black — you must choose the right method of removal.
Basic Steps for Killing and Removing Algae:
- Brush the pool walls vigorously to break up algae’s protective layer.
- Apply algaecide — preferably copper-based or quat-based for heavier strains.
- Shock with a chlorine-based shock (non-chlorine shocks are less effective on algae).
- Filter, backwash, and rebalance water chemistry.
Remember, algae often reappear if your pool’s sanitizer levels aren’t kept consistent.
4. Treating Mold and Mildew in Pools
Mold is more common in indoor pools or poorly ventilated outdoor spaces.
To treat mildew or mold on pool walls:
- Use a diluted bleach solution (1 part bleach to 10 parts water) but ensure no contact with eyes or skin surfaces.
- Apply directly onto the affected area and scrub with a stiff brush.
- Shock the pool and check air circulation around the pool area to prevent future accumulation.
Preventive Measures: Avoiding Future Brown Build-Up
The solution doesn’t end with cleaning. To keep your pool walls clean and prevent recurrence, consider the following:
1. Regular Maintenance and Cleaning Routine
This includes:
- Daily brushing of all walls and floors
- Consistent skimming of floating debris
- Weekly shocking (especially during warm months)
- Filter cleaning and backwashing as needed
2. Water Testing and Balance
Maintain a consistent water testing schedule:
Test your pool at least:
- 1–2 times weekly during heavy use months
- Every other week during off-seasons
Correct any imbalance immediately to prevent deterioration of surfaces and formation of stains.
3. Water Source Screening
If you’re filling your pool or adding water regularly:
- Check municipal water for high iron content
- Invest in a pretreatment system or iron filter if you use well water
Using a metal sequestrant can trap free iron and prevent staining.
4. Use of Stain Preventatives
Many pools benefit from the regular use of:
- Metal control chemicals
- Stain and scale inhibitors
- Maintenance algaecides
These products work proactively to maintain clarity and prevent staining or microbial growth before it starts.
5. Brushing and Circulation Optimization
Brown patches often appear in poor-circulation zones. Improve circulation by:
- Angling return jets upward and toward the walls
- Running the pump for 6–10 hours daily
- Using a pool cleaner to scrub all surfaces
When to Seek Professional Help
While most staining issues can be addressed with the right tools and approach, some situations warrant expert intervention.
Situations That Require A Pool Professional:
You’ve Tried Everything and Stains Return
If brown stains persist despite repeated cleaning and balancing, you might be dealing with:
- Structural damage beneath the surface
- Deep-rooted organic infiltration in porous material
- High levels of undetected metals in the water
Your Pool Has Severe Metal Buildup
High concentrations of iron or copper may require a complete water replacement or installation of a whole-pool filtration system. These are best handled by professionals equipped to test and treat for such issues.
Surface Damage From Improper Cleaning
Some pool surfaces — especially vinyl or fiberglass — can suffer from aggressive cleaning or acid washing. A professional can safely restore the material.
Conclusion: Keep Your Pool Inviting and Stain-Free
The brown stuff on your pool walls doesn’t have to be a mystery — nor does it have to persist. Whether it’s due to metals, algae, mold, or organic material, there is a safe, effective solution. Prevention through consistent testing and brushing is your best ally, but knowing the right treatment method is equally crucial when stains do appear.
Keep in mind, water balance, proper filtration, and proactive care go a long way in keeping your swimming pool a clean, beautiful, and healthy place to relax. By understanding what causes staining and how to manage it, you’ll be well on your way to a sparkling pool all season long.
If regular testing, brushing, or chemical care seems overwhelming, don’t hesitate to talk to a licensed pool service provider for a one-time evaluation or ongoing support. After all, a healthy pool is not only cleaner — it’s safer, more inviting, and more durable over time.
Now that you’ve found out, “What is the brown stuff on my pool walls?” you can confidently take the next step to address it — and keep it from coming back.
What causes the brown stuff on my pool walls?
The brown substance often found on pool walls is typically caused by one of three main factors: metals in the water, algae growth, or dirt and debris accumulation. When metals like iron or manganese are present in the pool water—often introduced through the source water or corroded plumbing—they can oxidize and form brown stains. These deposits stick to the pool surfaces and become more noticeable over time.
In addition to metals, brown algae can also appear on pool walls, especially in pools that are poorly maintained or have low sanitizer levels. Brown algae are more common in shaded areas and can blend in, appearing as a thin brown film. Another cause could be dirt or organic materials like soil and leaves that break down over time and settle onto walls, creating discolored spots. Identifying the exact cause is crucial to determine the right treatment method.
How can I tell if the brown substance is algae or metal stains?
Distinguishing between algae growth and metal stains can be achieved through a simple test. If you suspect the brown buildup is algae, apply a chlorine tablet directly to the stain or rub it with a cloth soaked in pool water. If the stain lightens or disappears, it’s likely algae. Algae tend to be more consistent in color and often feel slick or slimy to the touch, especially after a period of neglect in maintenance.
For a suspected metal stain, try applying a metal stain remover directly onto the affected section or place a small piece of ascorbic acid (vitamin C) in a sock and rub it over the brown area. If the discoloration fades, it’s very likely caused by oxidized metals. Metal stains tend to be more firmly attached and often appear in patches or streaks around waterline areas. Knowing the difference helps in choosing the right treatment product and avoiding further discoloration.
Can I clean brown stains myself, or should I hire a professional?
In most cases, brown stains on pool walls can be effectively cleaned by homeowners using do-it-yourself methods and products. If the stains are caught early and identified correctly, using pool-safe cleaners, brushes, and pH adjusters often leads to successful removal. DIY treatments are generally more affordable and readily available, making them a practical first step. Always remember to test a small area before starting a full clean to avoid damaging your pool’s surface.
However, if the staining is widespread, persistent, or the cause is unclear, it may be best to consult a pool professional. Experts can accurately diagnose the issue and suggest targeted treatments such as chemical shocking, acid washing, or even partial draining and resurfacing in severe cases. Seeking professional help can also prevent incorrect cleaning methods from worsening the problem or causing irreversible damage to vinyl, plaster, or tile surfaces.
What products are recommended for removing brown pool wall stains?
The right cleaning product depends on the cause of the brown stains. For metal stains, products containing ascorbic acid or citric acid are widely recommended because they effectively dissolve the oxidized metals clinging to the wall surfaces. There are also specialized metal stain removers like Jack’s Magic, Metal Free, or Zodiac Metal Remover, which are formulated specifically for pool environments.
For brown algae, chlorine-based treatments or algaecides tailored for mustard algae can be very effective. Additionally, using a pool brush to physically dislodge the algae is important alongside chemical treatment. Stain removing products should be used following the manufacturer’s guidelines to avoid damaging the pool’s finish or upsetting the water balance. Combining the right chemical treatment with thorough scrubbing typically offers the best results.
How often should I maintain my pool walls to prevent brown buildup?
Regular maintenance is key to preventing the formation of brown substances on pool walls. At a minimum, pool walls should be brushed at least once a week to dislodge any early forming algae or metal deposits before they take hold. More frequent brushing may be needed during periods of heavy use or high temperatures, when contaminants and organisms can multiply faster.
In addition to brushing, maintaining proper water chemistry—including balanced pH levels (7.2–7.8), adequate chlorine concentration (1–3 ppm), and regularly testing for metals—is imperative. A thorough inspection and wall cleaning should especially happen after heavy rain, when metals might enter the pool via runoff. Proactive maintenance helps preserve both the aesthetics and health of your swimming pool environment.
Are brown substances on my pool walls harmful to swim in?
While the brown substances on your pool walls aren’t immediately dangerous, they can eventually pose health and safety concerns. If the brown buildup is algae—particularly brown or mustard algae—it may indicate poor water sanitation, meaning other harmful bacteria could also be present. Stagnant water conditions combined with algae growth can reduce chlorine effectiveness, increasing the likelihood of irritation or infection for swimmers.
Metal stains and dirt accumulation are generally not harmful to touch or health, but long-term build-up can affect water clarity and promote microbial growth behind the deposits. Furthermore, stained areas can create rough or slick patches on pool walls and ladders, increasing the risk of slipping. For these reasons, addressing the brown discoloration as soon as it appears is vital to keeping the pool environment safe and enjoyable for all users.
What steps can I take to prevent future brown stains on my pool walls?
Prevention starts with consistent water testing and balancing, especially after filling the pool or introducing new water. If your water source contains detectable metals, consider using a metal sequestering or chelating agent to keep them in solution and prevent oxidation on pool surfaces. A steady chlorine residual helps avoid algae growth and keeps contaminants in check.
Good circulation and filtration are also crucial in preventing brown buildup; ensure the pump runs for an adequate number of hours each day. Routine brushing, regular vacuuming, and the use of a quality pool cover when the pool is not in use will help limit debris accumulation. Lastly, install a water softener or a metal filtration system if high mineral content is a persistent issue, to keep your pool walls consistently clean and clear.