Ultimate Guide to Swimming 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 fall out of 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.

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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.

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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.

For specific directions on treatment, you can check out our how-to articles for pool stain removal.

Organic Stains

At a Glance

Appearance

  • Light tan to greenish brown

Contributing factors

  • Debris sitting on pool surface
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Recommended Stain Removers

  • Pool Stain Treat®

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.

Product Recommendations

For organic stains, we recommend Pool Stain Treat®. For an isolated staining, you can use a Spotting Bag. For widespread stains we recommend using the 2lb. bottle.

Dosage:

  • 2 lb. bottle treats 20,000 gallons
  • Spotting bag treats a 1 foot radius

Pool Stain Treat®

Inorganic Metal/Mineral Stains

Metal pool stains can be trickier to remove. This is because certain metals are more likely to react to certain stain removing chemicals than others. This may require some trial and error before finding the best chemical to remove the particular stain.

In general, there will be two factors that directly impact how effective a pool stain treatment is: chlorine and alkalinity.

  1. Chlorine: As we mentioned, chlorine oxidizes metals and can even cause staining. Virtually all stain removers recommend lowering your chlorine reading in order to help dissolve the metal. High chlorine levels can interfere with most pool stain removing chemicals.
  2. Alkalinity: We also know that alkalinity is directly related to the solubility of metals. In general, lowering the alkalinity will help metals dissolve into the pool water. This means adding an acid to reduce alkalinity will typically speed up the stain removal process and yield better results.

Now let’s go over the most common types of metal stains and what chemicals work best at removing them.

Metal Flashing

At a Glance

Appearance

  • Water with color hue
  • Clear pool water

Contributing factors

  • High hardness
  • High alkalinity
  • Shocking

Recommended Stain Removers

  • Pool Stain Treat®

Description

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.

Product Recommendations

For metal flashing, we recommend Pool Stain Treat®. It will pull the metals back into solution and make the water a beautiful blue again. We also recommend it for prevention: just add half a bottle (1lb.) once a month to prevent metal flashing.

Dosage:

  • Stain Removal: 2 lb. bottle treats 20,000 gallons
  • Prevention: 1lb. per 20,000 gallons every 30 days.

Pool Stain Treat®

Pool Scale

At a Glance

Appearance

  • White chalky buildup

Contributing factors

  • High hardness
  • High alkalinity

Recommended Stain Removers

  • No Drān® Acid Wash

Description

While we don’t typically think of calcium scale as a metal stain, it absolutely is. Scale is calcium carbonate deposits that have formed on the surface of the pool. 

The most direct way to remove scale below the pool water line is by lowering the alkalinity so the pool water will dissolve it. Certain chemicals will also make calcium very soluble in pool water. Sulfamic acid is a great descaler that can remove calcium deposits quickly while breaking down carbonates. 

Iron Stains in Pools

At a Glance

Appearance

  • Light tan to reddish brown

Contributing factors

  • Well water source
  • Fertilizer around pool
  • Shocking
  • High Alkalinity
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Related How-Tos

Recommended Stain Removers

  • Pool Stain Treat®

Description

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.

Silver Stains in Pools

At a Glance

Appearance

  • Light gray to black

Contributing factors

  • Silver algaecide
  • Shocking
  • High alkalinity
e

Related How-Tos

Recommended Stain Removers

  • Pool Stain Treat® or Super Stain Treat™

Description

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.

Product Recommendations

For iron and silver pool stains, we recommend Pool Stain Treat®.  We also recommend it for prevention: just add half a bottle (1lb.) once a month to prevent metal stains from returning.

Dosage:

  • Stain Removal: 2 lb. bottle treats 20,000 gallons
  • Prevention: 1lb. per 20,000 gallons every 30 days.

Pool Stain Treat®

Manganese Stains in Pools

At a Glance

Appearance

  • Light purple to black

Contributing factors

  • Well water source
  • Shocking
  • High alkalinity

Recommended Stain Removers

  • Super Stain Treat™

Description

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.

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

Copper Stains in Pools

At a Glance

Appearance

  • Light green to dark blue

Contributing factors

  • Copper plumbing
  • Copper heater
  • Copper algaecide
  • Shocking
  • High alkalinity
e

Related How-Tos

Recommended Stain Removers

  • Super Stain Treat™

Description

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).

Product Recommendations

For copper and manganese pool stains, we recommend Super Stain Treat™.  After the stains have been removed, you can switch to Pool Stain Treat for prevention.

Dosage:

  • Super Stain Treat™: 2.5 lb. bottle treats 15,000 gallons
  • Pool Stain Treat®: 1lb. per 20,000 gallons every 30 days.

Super Stain Treat™

Pool Stain Treat®

Cobalt Stains in Fiberglass Pools

At a Glance

Appearance

  • Light gray to black

Contributing factors

  • Older fiberglass
  • Cracked resin

Recommended Stain Removers

  • Super Stain Treat™

Description

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.

Product Recommendations

For cobalting in fiberglass pools, we recommend Super Stain Treat™.  After the stains have been removed, you can switch to Pool Stain Treat for prevention.

Dosage:

  • Super Stain Treat™: 2.5 lb. bottle treats 15,000 gallons
  • Pool Stain Treat®: 1lb. per 20,000 gallons every 30 days.

Super Stain Treat™

Pool Stain Treat®

Preventing Stains

Now that we’ve gone over the potential stains you may have to deal with in your swimming pool, let’s look at how to prevent them. Prevention is not only easier, but it’s more cost effective as well.

Preventing Organic Stains

Preventing organic stains are pretty simple: don’t let debris settle on the bottom of your pool. 

You can do this by reducing the amount of debris that enters. Just adjust some of the landscaping around the pool, and keep trees well trimmed.

You can also reduce the odds of debris settling: 

Adjust Returns

You can adjust return lines to rotate the top of the water in a counterclockwise direction towards your skimmer. This will take advantage of the coriolis effect and trap debris in your skimmer before it settles.

Skim more often

You can also simply skim the surface of the water more frequently with your pool net.

Install an Automatic Cleaner

Finally, you can install an automatic cleaner that will vacuum up the debris.

Preventing Metal Stains

Preventing metal stains in your pool comes down to your water chemistry and how you balance. Employing these suggestions will help prevent your pool from getting stains.

Lower Alkalinity

First and foremost, you will need to balance the alkalinity according to your water hardness. As the water gets harder (and thereby contains more metals) the alkalinity will need to be lowered. You can learn about our specific recommendations by downloading our Hamilton Index™.

Avoid Shocking

You’ll also want to avoid shocking as much as possible. The more often you shock, and in greater amounts, the more likely metals in the water will become oxidized and form stains. Shocking can be necessary, so just make sure to only do the minimum required to get the job done.

Balance in smaller increments

Use pH increaser in smaller increments. Most pH raisers contain carbonates, and if they react to the metals they can form stains. Using less at a time will reduce the chances of the water becoming “overloaded” and dropping metals.

Dissolve it first

If you’re adding anything granular to your pool, try dissolving it in a bucket of water first. This will dilute it’s immediate concentration (it won’t have an impact on the “net” effect of the chemical), and will reduce the chances of anything settling on the bottom that can form stains.

Always brush after

Anytime you add a granular pool chemical, make sure to follow it with brushing. Brushing will help any granules that may have settled get kicked back up into the water to dissolve further.

We hope you enjoyed our Ultimate Guide to Pool Stains. If you need any extra help feel free to contact us by opening a support ticket.

FREE DOWNLOAD

Frustrated with stains?

Use our cheat sheet to diagnose your stain and see which product will remove it!

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2 Comments

  1. Pearl

    Wow! We did not know that there were these different types of pool stains. We just referred to them before as “this” pool stain and “that” pool stain. Haha! Really helpful tips by the way.

  2. Scott Hamilton

    Thanks pearl! The good news is they are mostly similar so what works for one often works for another =)