Few surfaces can match the sleek finish and smooth touch of fiberglass.

However, nasty streaks of brown or turquoise can quickly mar any poolscape. Oxidized metals are the common culprits behind these unsightly marks.

The good news is that stains on fiberglass pools are exceptionally easy to clean.

“Fiberglass is chemically inert,” says Alan Schuster, president of Aqualab Systems in Apex, N.C. “In that respect, it’s easier to maintain because it doesn’t affect the pH. Also, it doesn’t have little nooks and crannies for algae to grow.”

The bad news? The fiberglass shell and the offending metals have an affinity for one another. The negative charge of fiberglass, which comes from ground static, attracts positive metal ions. As a result, the shell’s nonporous structure reveals stains more quickly.

To effectively battle these stains, it’s important to identify not only which metal you’re dealing with, but also where it comes from. Whether it’s iron, copper, calcium or manganese, you’ll need to treat the water accordingly to keep your fiberglass in pristine condition.


What it looks like: Iron is familiar to most service technicians, manifesting itself in ugly streaks and skids across the pool wall. In worst-case scenarios, a powerful sanitizer such as dichlor may oxidize the iron, transforming the entire pool into a murky brown mess.

Iron staining is typically darker in color, most often identified by a reddish brown tint. It may also appear as black, gray or yellow marks.

What causes the stain: The most common cause of iron-heavy water is source water. Though what comes out of the faucet may be clear, changing pH values can bring iron to the fore.

“Under acidic conditions, the [metal] is soluble and you don’t realize you have a problem,” Schuster says. “Then you raise the pH and all hell breaks loose.”

In older pools, iron piping is another source of metal staining. High flow rates and corrosive water can deteriorate worn-out plumbing and introduce additional iron into circulation. Old valves and pump impellers may corrode as well, further adding to the pool’s iron content. If the customer agrees to the cost, replacing old equipment could be the best remedy against iron staining.

Chemical buildup is another often-overlooked source of iron because the metal is present in many of the chemicals that service techs use for maintenance.

“When it’s yellow, common pool acid may have a higher contaminant level, and a lot of times that’s iron,” says Jack Beane, president ofJack’s Magic Products in Largo, Fla.

Beane adds that the quantities of chemicals used are small, but when new water is not introduced, the buildup can manifest itself in these red and brown stains. In addition to liquid chlorine, some grades of calcium chloride have high iron content as well.

How to treat and/or prevent an iron stain: To treat these stains, use a sequestering or chelating agent, which helps hold the iron in solution and create filterable particles.

“On average, we’re adding about 5 ounces [of sequestering agent] a week to a 10,000- or 15,000-gallon pool,” says Todd Starner, president of the Independent Pool & Spa Service Association’s Manasota Chapter in Florida.

If source water is the culprit, reintroduce this stain-guarding agent anytime the pool is refilled with fresh water. Checking for metals is done through a simple colormetric test, available from most major manufacturers. Just make sure you test the source water as opposed to the pool water. Oxidized metals that have come out of solution will not show up on the test.


What it looks like: Manganese is a less frequent source of staining, but it can be just as noxious as high levels of iron or copper.

Manganese stains can take on a variety of colors, including brown, purple, pink and black.

This staining is usually tied to specific regions.

What causes the stain: “With manganese, we see it predominantly on pools that are up on wells, which are typically in the mountains,” notes Javier Payan, owner of Payan Pool Service in El Cajon, Calif. “It usually manifests itself at the pool’s fill line.”

Manganese is relatively uncommon in the United States, though Schuster says pools on the south shore of Long Island are susceptible. Service techs in south Texas have come across manganese staining as well.

“Also, if you corrode stainless steel, you could be introducing manganese,” Schuster says. “But stainless steel is fairly resistant.”

Potassium permanganate-based water treatment filters can be a source of manganese, too, Beane says. The filters remove iron, but introduce manganese through an ion exchange.

How to treat and/or prevent a manganese stain: Again, a sequestering agent that targets manganese may be the best way to protect against these stains.

Try increasing the dosages because other metals typically accompany the presence of manganese.

Few dealers test for this metal, but colorimetric test kits are available from several major manufacturers.

As always, the best defense against any metal stain is controlled water chemistry and regular use of metal-specific sequestering or chelating chemicals. These stain-guarding agents are especially important in pools with highly metallic source water.

And remember to balance your chemistry after dosing a sequestering agent to ensure the metals stay in solution.


What it looks like: Many service techs don’t think of calcium as a metal, but check your periodic table. As with other metals, it can easily come out of solution and mar the surface of a pool.

Regular calcium scale is rare in fiberglass pools, but it can still appear as a familiar white crust.

“We do see calcium buildup on vanishing edges and spa spillways,” Payan says.

More often, calcium combines with other metals to create particularly resilient stains. These metal salts can cause staining in a variety of colors, making them difficult to identify.

“The colors fluctuate,” Beane says. “What you think is a common iron stain could be iron scale. And it’s two different processes to treat them.”

As such, he recommends using a stain identification kit to determine which metal the scale has combined with.

Stain ID kits use basic oxidation-reduction to create reactions to certain metals. It’s important to note that correct water balance is essential when using these products.

What causes the stain: Naturally, high calcium hardness is part of the problem, combined with high pH and alkalinity readings. Recommended levels of calcium range between 200- and 400 ppm, per the >National Swimming Pool Foundation’s Certified Pool-Spa Operator’s Handbook.

However, because fiberglass pools do not have a masonry finish, the calcium hardness level doesn’t need to be as high.

“[In fiberglass pools], most dealers keep the calcium lower, closer to 80 ppm,” Schuster says. “If you have 200 ppm, you close the door to using cal hypo as a shock.”

Using disinfectants such as cal hypo will boost calcium hardness levels, especially in areas with high temperatures and water evaporation. High calcium also can be found in source water, particularly in the West and Southwest.

How to treat and/or prevent a calcium stain: Most service techs use calcium hardness to help determine a pool’s Saturation Index. Even so, at lower hardness levels, a sequestering agent will help keep the calcium in solution. In addition to controlled calcium levels, sequestering should prevent the formation of metal salts.

To measure calcium hardness, use a titration test. If the end point (expected color change) cannot be achieved using a reagent, it may indicate the presence of other metals. Purple clumping also may point to the presence of manganese.

Depending on the reagent, manufacturers should provide corrective measures for accurate hardness readings. But, at the very least, a failed calcium hardness test could lead to the discovery of other stain-causing metals.


What it looks like: Copper may be the most common stain, perhaps because it has so many different sources.

Its telltale turquoise color can appear in any pool, regardless of style or region. Copper also may appear black or gray, depending on the finish.

What causes the stain: As with other metals, checking source water is a good place to start. High-copper source water is usually found in areas that utilize surface water, such as lakes or reservoirs. Because copper is such an effective algaecide, it is a favorite choice for keeping these water sources clean.

Like iron, copper is found in older plumbing. However, a stripped copper heat exchanger is a more likely source. As seen with iron, this deterioration can result from high flow rates and corrosive waters. Check the heater for an acceptable flow rate.

“Pools with copper plumbing are going to be prone to copper sulfate — those turquoise stains you get,” Payan says. “The [two] things that accelerate [staining] are putting tablets in the skimmer and having an inline chlorine feeder.”

Payan cites a common problem in residential pools, where a chlorine tablet — typically trichlor, which is highly acidic — is left in the skimmer. During circulation, the sanitizer is eroded and distributed to the pool evenly. Once the pump is shut off, the tablet builds a pool of chlorine concentration.

In the morning, when circulation typically resumes, the chlorine concentration is immediately pulled through the skimmer and into the circulation system. The corrosive nature of this “chlorine pocket” can easily break down old piping and copper heat exchangers.

How to treat and/or prevent a copper stain: Be sure your flow is consistent with heater requirements. Copper stains often result from a stripped heat exchanger, particularly in commercial spas. Though some trichlor products are designed for skimmer use, it’s important to keep the tabs out of the skimmer unless a manufacturer recommends it.

“With commercial [spas], the chemical fluctuation changes so dramatically based on usage,” Payan notes. “It’s a guessing game and … a chlorine spike can start wearing away at the heat exchanger.”

Finally, any pool using ionizers will show copper traces. Most ionizer manufacturers recommend a copper level between 0.1 and 0.3 ppm. Anything more could fall out of solution.

Though a sequestering agent should be used, testing the source water remains vital.

“At the end of the day, people should have their water tested for minerals,” Schuster says. “You have to know what you’re up against.”