If you’ve noticed a stain in your fiberglass pool, you’re not alone, and it doesn’t always mean something is seriously wrong.
Stains can happen for a variety of reasons. Leaves may sit too long, metals can make their way into the water, or your water chemistry may drift out of balance. The good news is that most fiberglass pool stains are manageable, and many aren’t permanent.
Before you reach for a cleaner, the most important step is figuring out what kind of stain you’re dealing with. Once you know that, choosing the right solution becomes much easier.
What types of stains occur in fiberglass pools?
Most pool stains fall into two main categories: organic and inorganic.
Organic stains come from natural materials like leaves, dirt, or berries. These typically sit on the surface and are often easier to remove.
Inorganic stains are caused by metals or mineral buildup in the water. These can be a bit more complex and may require specific treatments depending on the source.
What are organic pool stains?
Organic stains usually form when debris sits in the pool for too long. Common causes include leaves, dirt, and other natural materials.
These stains tend to stay on the surface and, in many cases, will fade over time or respond well to brushing.
It’s worth noting that algae isn’t technically a stain, but certain types, like black algae, can look similar and require a different treatment approach.
What are inorganic pool stains?
Inorganic stains are typically caused by metals or minerals in the water. These include metal stains, calcium scaling, and cobalt staining.
Metal stains
Metal stains can come from your water source, pool equipment, or introduced metals.
Common types include:
- Iron (orange, brown, or tan discoloration)
- Copper (green or blue tones)
- Manganese (brown, black, or purple hues)
While less common in modern fiberglass pools, they can still occur if metals are present in the water.
Calcium scaling
Calcium scaling isn’t technically a stain, but it behaves like one and is treated similarly.
It shows up as a white or gray buildup on the surface. There are two types:
- Calcium carbonate, which is flaky and reacts to acid.
- Calcium silicate, which is harder and more difficult to remove.
Cobalt staining
Cobalt staining appears as dark or black spots on the pool surface. Despite how it looks, it’s not actually a surface stain.
It’s caused by a reaction beneath the gelcoat when water interacts with certain materials. Because of this, it doesn’t respond to the same treatments as typical stains.
How can you prevent stains in a fiberglass pool?
As with most pool problems (and life problems, for that matter), it’s best to prevent stains in the first place rather than clean up the mess later.
The most important factor is maintaining proper water chemistry. Balanced water helps prevent metal staining, calcium buildup, and organic discoloration from setting in.
Regular cleaning also makes a big difference. Removing leaves, dirt, and debris before they sit too long can prevent staining, especially around the waterline.
A consistent routine goes a long way in keeping your pool looking clean.
How do you remove stains from a fiberglass pool?
Let’s say, theoretically, you have a li’l whoopsie-daisy and a stain shows up in the pool. Have a good cry (I won’t judge), and then we’ll get down to business.
Let’s go basic: see if you can wipe it off. Yes, this is the “have you tried turning it off and on again?” solution of pool cleaning. Yes, you should still do it.
Use a soft sponge/fabric and products that are specific to fiberglass pools. Using harsh chemicals or rough tools could damage the gelcoat.
If the stain remains, your next step depends on the source, the type of stain. Time to use that water testing kit we talked about.
Removing organic stains
Organic stains are usually the easiest to handle.
In many cases, regular brushing will gradually remove them. If that doesn’t work, a fiberglass-safe stain eraser can help lift the discoloration.
If the stain is still visible, time is often your ally. Many organic stains fade naturally within a few weeks to a couple of months.
Removing nonorganic stains
Metal stains
Metal stains require a more targeted approach.
For small spots, rubbing a vitamin C tablet directly on the stain can help release the metal from the surface.
For larger stains, you’ll need to treat the water:
- Lower chlorine to 0.0 ppm and adjust pH to around 7.2
- Add ascorbic acid (about one pound per 10,000 gallons)
- Run the system for 24 hours
- Rebalance the water
- Add a metal sequestering agent
This process removes the stain and helps prevent the metal from redepositing elsewhere.
Calcium scaling
Calcium buildup depends on the type you’re dealing with.
Calcium carbonate can often be removed with a stain eraser or a fiberglass-safe scaling treatment.
Calcium silicate is more stubborn and typically requires a dedicated treatment. In some cases, a pool professional may be needed for faster results.
Cobalt Staining
Cobalt staining can be frustrating because it looks like a typical stain, but it doesn’t behave like one.
Unlike organic or metal stains, cobalt staining happens beneath the gelcoat due to a chemical reaction. Because of this, standard stain removal methods like brushing, vitamin C, or ascorbic acid treatments won’t be effective.
In most cases, surface cleaning won’t remove cobalt staining.
The best first step is to have the area evaluated by a pool professional who is familiar with fiberglass pools. In some situations, the discoloration may improve over time, but in others, more advanced solutions may be needed.
The key takeaway is this: if a dark spot isn’t responding to typical stain treatments, it may not be a stain at all, and treating it like one can lead to frustration (and wasted effort).
What should you avoid when removing fiberglass pool stains?
When treating stains, it’s just as important to avoid damaging the pool surface.
Avoid using abrasive brushes or harsh chemicals that aren’t designed for fiberglass pools. Scrubbing too aggressively or using the wrong products can damage the gelcoat, which protects the pool’s finish.
What’s the best way to handle fiberglass pool stains?
Most fiberglass pool stains aren’t permanent, and many are easier to deal with than they first appear.
The key is to identify the type of stain, use the right method to remove it, and stay consistent with basic maintenance. Keeping your water balanced and your pool clean will prevent most staining issues before they start.
And when something does pop up, you’ll know exactly how to handle it.
Frequently Asked Questions
What are the different types of fiberglass pool stains?
Fiberglass pool stains fall into two main categories: organic and inorganic. Organic stains come from natural materials, while inorganic stains include metal stains, calcium buildup, and cobalt staining.
What causes stains in a fiberglass pool?
Stains in a fiberglass pool are usually caused by organic debris like leaves and dirt, or inorganic sources like metals and minerals in the water. Unbalanced water chemistry can also increase the likelihood of staining.
How do you remove stains from a fiberglass pool?
Start by wiping the stain with a soft cloth or sponge using fiberglass-safe products. If it doesn’t come off, the next step depends on the type of stain—organic stains may brush out or fade over time, while metal and calcium stains require more targeted treatments.
How do you remove metal stains from a fiberglass pool?
Small metal stains can often be removed by rubbing a vitamin C tablet directly on the spot. Larger stains typically require an ascorbic acid treatment followed by adding a metal sequestering agent to the water.
Can you remove cobalt staining from a fiberglass pool?
Cobalt staining cannot typically be removed with standard cleaning methods because it occurs beneath the gelcoat. If a stain isn’t responding to normal treatments, it’s best to have it evaluated by a pool professional.
About the Author
Pool stains can be frustrating, especially when you’re not sure what caused them or how to fix them.
I’m Cristian Shirilla, Marketing Manager at River Pools and co-owner of a fiberglass pool installation company in Virginia. Through my work with homeowners and pool professionals, I’ve seen how often stains come down to a few key factors like water chemistry, debris, or misidentified issues.
I wrote this article to help you quickly understand what you’re dealing with and how to handle it the right way, without damaging your pool or wasting time on the wrong solution. My goal is to make pool ownership feel a little more manageable (and a lot less guesswork).
From the Manufacturer
At River Pools, we regularly hear from homeowners who notice a stain in their fiberglass pool and aren’t sure what caused it or how to fix it.
In many cases, staining comes down to factors like water chemistry, debris, or metals in the water, and once you identify the cause, it’s usually manageable. Resources like this are designed to help you understand what you’re dealing with so you can take the right approach without guesswork.
If you’re considering a fiberglass pool, the next step is connecting with a trusted independent installer who can walk you through your options and what to expect.
Ready to start the conversation?
