Ultimate Guide to Pool Stains

Found a blemish on your swimming pool surface and not sure what it is? Stains can be one of the most frustrating pool problems you’ll run into. In this ultimate guide, we give you the complete breakdown of pool stains and how to get rid of them.

Types of Swimming Pool Stains

While there are quite a few different kinds of stains you may run into, we can classify all stains into two types: organic and inorganic

Organic Pool Stains

“Organic” in chemistry simply means a chemical that contains carbon.

In more practical terms, a stain is organic if it is the result of something that was alive. These include things like leaf stains, algae stains, stains from acorns, tannins from tree bark, and other pigments left from organic matter. When this organic matter sits on the surface, some of the pigments get transferred to the surface to form stains.

The biggest factor of organic pool stains is what’s surrounding the pool. As more debris accumulates in the pool, the more likely it will leave organic stains.

To remove them, the complex carbon chains of the molecules must be broken apart.

Inorganic/Metal/Mineral Pool Stains

Inorganic stains are different metals that form deposits on the surface. Metals present in the pool water will fall out of the solution and chemically bond to the surface. And in some cases, can even leach from the surface.

The biggest contributing factor of metal staining is balancing – specifically the Total Alkalinity. This is because alkalinity measures carbonates in your pool water, which is important for stabilizing pH. However, when metals react to carbonates they become insoluble in pool water – leading them to form deposits. So the higher the alkalinity, the more likely you are to get metal staining.

The trick to removing metal staining is finding the right chemical to react with. This will help the pool water dissolve it.

Diagnosing Swimming Pool Stains
Determining the type of stain is relatively easy. In general, organic stains respond to chlorine while metal stains do not. Chlorine oxidizes the carbon breaking apart the pigment molecules and lightening the stains.

Chlorine will oxidize metals as well. But this actually makes the stain less soluble. In fact, shocking is one of the biggest causes for metal stains.

So if after shocking the pool the stains lighten, it’s likely organic. Otherwise, it’s usually a metal stain.

Metal Testing Kits can be unreliable

Metal tests are a common go-to diagnostic tool for pool owners and local pool stores alike. However, metal tests can be unreliable when diagnosing stains.

First, tests only react to metals in the pool water. If the metals are on the surface then they won’t be present in the sample. Then there’s no way they can react to the reagents, giving a false negative.

The reagents themselves are also dependent on the particular state and compound the metal is in. This can lead to the reagent missing certain traces of the metal, also giving a false negative.

So if a particular metal test comes back negative, it doesn’t mean that the particular metal isn’t present per se.

Removing Pool Stains

Now that we understand the different types of stains, how to diagnose them, and what precautions to take with the pool surface we can begin to look at methods of pool stain removal.

Organic Pool Stains

At a glance

  • ULight tan to green
  • PDebris was sitting on surface
  • PRecent algae growth in area of stain

About Organic Stains

Organic stains are the easiest to treat. Simply by raising the chlorine level and/or shocking the pool, you can break apart the carbon chains of the various pigments. This coupled with brushing will usually make quick work of any organic pool stain.

You can also use oxalic acid as a means to remove organic pool stains. Being a strong oxidizer, oxalic acid will also break up the carbon structure of most organics.

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Metal Flashing

At a glance

  • UClear water with greenish hue
  • PHigh Shocking
  • PHigh Alkalinity

About Metal Flashing

While metal flashing itself isn’t a stain, it can be an early indicator of staining. Metal flashing is where metals come slightly out of the pool water – but not completely. Often this will cause the pool water to change colors – most often green. 

The water itself will remain clear. However, the metals refract the light bouncing in the pool water turning it a different hue. This usually happens when adding a large amount of chemicals at one time to the water – especially when adding chlorine.

If you notice metal flashing it’s a clear sign you have high metals. You should immediately stop adding the chemical you are currently and check your balancing. It can usually be corrected by simply lowering your Total Alkalinity to compensate for the high metal content.

You can also add a chelating agent to the water. This will bind the metals in water soluble compounds and prevent them dropping out to stain again.

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Copper Pool Stains

At a glance

  • ULight Green to Dark Blue
  • PCopper Heater
  • PCopper Plumbing
  • PCopper Algaecides
  • PHigh Shocking
  • PHigh Alkalinity

About Copper Pool Stains

Copper is also a fairly abundant metal that can form stains in swimming pools. Depending on the state of the copper, stains can appear as green, blue or even black. Copper primarily comes from two places: copper algaecides and copper plumbing.

Copper is a common ingredient in many metal based algaecides, and copper has been used as a disinfectant for thousands of years. While copper sulfate itself is pool water soluble, it can react to other compounds to form stains. The most common scenario is using copper sulfate followed by shocking. The shock causes the copper to become oxidized and fall out of the pool water, thus staining the surface.

Copper can also leach from plumbing and equipment. The most common source of this is older copper pool heaters. If the water is too acidic, trace amounts of copper enter the water. However, this is becoming less common as alternative metals – like titanium – are being used for newer pool heaters.

Ascorbic acid (aka Vitamin C) is very effective on copper. Ascorbic acid also usually won’t react to dyes on the surface of the pool. This makes it safe to use on any surface without diluting. Ascorbic is also a chelator, so it will help bind the metals in the water (although not quite as well as oxalic acid).

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Iron Pool Stains

At a glance

  • UTan to dark red brown (rust)
  • PWell water
  • PFertilizer
  • PHigh Shocking
  • PHigh Alkalinity

About Iron Stains in Pools

Rust stains are one of the most common types of pool stains. This is because iron is one of the most abundant minerals you’ll find around pools. Wells typically are high in iron content, as well as soil and fertilizers surrounding the pool. 

In older homes galvanized iron pipes may be used. This too can lead to iron entering the pool.

Also if any of the rebar of plaster pools is exposed to the pool water, the rebar can rust forming iron stains. Usually this only appears with cracked pools or pools that were installed poorly.

As you would expect, iron forms light tan to dark red-brown stains in the pool, similar in color to rust.

Oxalic acid is an extremely effective iron stain remover. It reacts extremely well to iron and forms double bonded pool water soluble compounds (referred to as “chelation”). The iron is then trapped in these dissolved compounds where they’re less likely to form stains again.

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Silver Pool Stains

At a glance

  • ULight gray to black
  • PSilver algaecide used
  • PHigh Shocking
  • PHigh Alkalinity

About Silver Pool Stains

Silver stains are fairly uncommon. You should only run into silver stains after using a silver based algaecide. Like copper, silver will become insoluble after a heavy dose of shocking and form grey-black stains on your pool.

Both Ascorbic and Oxalic acid will react with silver. However, silver is not very reactive and it may take quite a bit of stain remover to get it off your pool.

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Manganese Pool Stains

At a glance

  • ULight purple to black
  • PWell water source
  • PHigh Shocking
  • PHigh Alkalinity

About Manganese Pool Stains

Manganese is also a common metal, especially with pools that use well water. Manganese is also common in fertilizers. Manganese is highly reactive, forming “permanganates” when oxidized.

Manganese can appear as anything from purple stains in the pool, to even a dark brown black color. It some cases, the water itself can turn a magenta to purple color.

Manganese will respond to both oxalic acid and also ascorbic acid, usually with better results using ascorbic.

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Cobalt Stains

At a glance

  • ULight Gray to Black
  • POld Fiberglass Pool
  • PAppears consistently in same spot
  • PCracked Resin

About Cobalt Stains

Cobalt usually isn’t a problem in most pools. However, cobalt is sometimes used as a hardening agent in fiberglass resins. 

Older fiberglass pools with high cobalt content in the resin will sometimes leach the cobalt out into the pool water, forming a stain. Although not particularly attractive, this doesn’t compromise the integrity of the fiberglass resins per se.

Cobalt appears as grey-black spots on the surface of the pool. Typically the stained area will have a slight bump where the cobalt is leaching out.

Since this stain originates from the surface, you’ll need to leach it out of the water faster. As counterintuitive as this may seem, the stain only forms when the water stops dissolving it.

Both Oxalic and Ascorbic can help dissolve the surface cobalt. Unfortunately, deposits will continue to form as long as there is cobalt present. So it’s strongly recommended you continue to use a stain remover to keep cobalt stains at bay. If the problem is persistent enough, consider resurfacing the pool with a high quality gel-coat.

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