Stains are a common occurrence in swimming pools, and their removal can be challenging. What works well on one stain may not be nearly as effective on the next. No two pool owners will describe the same stain in the same way. What appears brown to one pool owner will appear green to the next.

The first rule of stain removal is to test the stain. The second rule is never give guarantees. This manual provides information on stain removal and prevention in pools, including the causes of stains and testing methods, as well as stain removal techniques.

What are stains?

A stain is something that alters the appearance of the pool surface to the detriment of the pool. This definition includes scale and most pool owners would indeed call scale a stain. Scale is usually coloured because of the inclusion of metals and dirt. In the strict sense, scale is not a stain but a deposit of Calcium Carbonate (chalk) on the pool surface, which can be recognized by the milky appearance and by the rough surface (often referred to as like sandpaper).

What causes stains?

Stains are most commonly caused by the introduction of metals into the water which react with Chlorine or the slightly alkaline pH to give dark coloured metal oxides which we call stains. Metals can be introduced by:

    • Salt added to a salt water pool – Salt is the most common cause of stains. Usually Iron which depending on the severity, gives green through yellow to brown stains or Manganese which gives dark brown to black stains.
    • Liquid Chlorine – Whose yellow colour is usually Iron. After continuous use for years the Iron build up can cause yellow to brown stains.
    • Top-up water – Especially bore water which can contain Iron, Manganese or Copper.
    • Algaecides containing metals – May include Copper, Silver and/or Nickel, all of which give black stains.
    • Metal fixtures – Especially in Above Ground pools where the ladder and walls are made of cheap stainless steel which give Iron stains.
    • Garden dirt – Washed into the pool after heavy rain or watering the garden. May contain Iron, Manganese, Vanadium, Copper, to name a few.
    • Black Spots in Fibreglass Pools – The Black Spots are Cobalt Oxide stains. The Cobalt comes from osmosis of the gel coat which leeches some of the Cobalt catalyst from the fibreglass (polyester) resin. Osmosis is also characterised by blisters at the site of the black spot.
    • Organic stains – From Algae growth, leaf stains, or dirt washed into the pool and left longer than a week. Usually yellow to green.
    • Oxidation – Of fibreglass or painted surfaces which give a milky or chalky appearance. Oxidation is not just the result of high Chlorine levels but high Chlorine levels combined with a low pH. The use of Trichlor Tablets, which have both a high Chlorine content and a low pH, needs special care!
    • Scale – Deposited on the surfaces of the pool giving a chalky or milky appearance and a sandpaper-like feel. Scale is caused by: Water out of balance from too high pH, too high Total Alkalinity, too high a Calcium level, or too high T.D.S.

Identify the cause:

To avoid the frustration of removing a pool stain, only to see it reappear after a few weeks, it is important to identify the cause of the stain. For example, salt is a major cause of stains in pools, and pool owners must realise that stains are an ongoing problem in salt pools.

Ask yourself the following questions;

    • Did the staining appear after adding salt?
    • Was Copper based Algaecide used in the pool?
    • Do they have an Ioniser?
    • Do they have a leaf problem and do leaves sit in the pool for weeks?
    • Has there been a storm and dirt has been washed into the pool?
    • How long has the stain been there? 
    • Have they tried anything themselves?

Stains broadly fall into four categories:

Organic stains:

Leaf Staining

Most commonly leaf stains, where a leaf has been allowed to sit in one place for weeks and Algae stains where the pool was allowed to stay green for weeks. Dirt also gives rise to organic stains. Dirt can be washed in the pool after a storm or if the hydrostatic valve operates and dirty water enters the pool (characterised by a discoloured ring around the main drain).

The most common colours are yellow, green and brown.

Metal stains:

Copper staining in pool

Most commonly Iron which gives yellow through to brown stains and Black Spots in Fibreglass pools which are Cobalt stains.

Oxidation of fibreglass and painted surfaces:

Osmosis Fibreglass Pool

This is a chemical reaction that occurs primarily in fibreglass and painted concrete pools. It causes the surface of the pool to degrade resulting in a faded, discoloured, or dull finish over time. 

The reaction is generally caused by a combination of incorrect water balance, and exposure to UV radiation, and high Chlorine levels. This causes the surface material of the pool to break down and appear faded.

Prevention is key when it comes to oxidation, regular maintenance and water testing by a pool professional is the best way to safeguard your investment, and prevent the need for costly resurfacing.


Calcium scale in pebblecrete pool

This is the build up of minerals such as Calcium and Magnesium on the surface of the pool, basically when the hardness level in your pool exceeds the saturation point the minerals can fall out of solution and deposit a layer of scale on the pool surface or inside the plumbing.

Maintaining proper water chemistry is key to avoiding scale in the first place, as always the professionals at Clarity Pool Management in Sydney are always here to help.

Testing The Stain

Testing the stain will determine what caused it so you can prevent it, what the stain is made of, and how long it will take to remove it or if it is possible to remove the stain completely.

Testing for Iron, Manganese, Cobalt, Silver , Organic stains and Scale:

Place a handful of Lo-Chlor Multi Stain Remover in a skimmer sock and attach it to your pool to put pressure on the sock. The skimmer sock is then wiped, in a straight line; backwards and forwards through the stain. It is best to pick the worst part of the stains.

    • Iron stains and Organic stains will be removed instantly leaving a clear stripe.
    • A Manganese stain will be removed within two minutes.
    • Black spot in fibreglass pools will also be removed in two minutes.
    • Finally a Copper or Silver stain will take five minutes to disappear using this method. 
    • Scale may be lifted a little by this test and the original colour of the pool may show through. The result depends on how thick the scale is or how long it has been there.

Testing specifically for Copper:

A handful of Lo-Chlor CU Later Part-I and Part-II is put into a skimmer sock, and using the pool broom; the above test is repeated.

    • A Copper stain is removed within two minutes provided the stain has been there less than one month.

Note: You cannot test vinyl surfaces this way. There is a risk of the acidity of the Nail the Scale, bleaching the vinyl.

Copper is absorbed into the surface over time so it is important to get to the stain as fast as possible. After one month there is little hope for a marblesheen or pebble surface and the pool must be emptied and acid washed.

After six months the Copper will have absorbed so far into the surface that an acid wash may only remove the surface stain and the stain may reappear after a period of time.

Do not give guarantees with Copper stains!

Testing for organic stains:

A handful of granular Chlorine is put into a skimmer sock and the above test is repeated. If there is no effect within a minute let the sock sit in one place for three minutes.

    • Organic stains should be removed within three minutes.

Stain Removal

Having carried out the above test you should have determined what has caused the stain and you can follow the below guides to remove the stain.

Removing the stain from the surface of the pool is half the battle. The material that caused the stain is still there but it is suspended in solution. It is very common to remove a stain only to have it reappear after three weeks or two months. In order to avoid this you must sequester the metals using a metal remover allowing the filter to trap them.

Before any stain removal the Chlorine level in the pool must be removed. Chlorine will neutralise most stain removers making them less effective.

In the middle of summer it may be sufficient to turn off the salt chlorinator for two days beforehand or not add any Chlorine for two days. If you are in a rush a Chlorine removal agent can be used.

In winter Lo-Chlor Chlorine Remover will be necessary. 250 Grams of Lo-Chlor Chlorine Remover will remove 5.0 PPM free Chlorine in 50,000 Litres in two hours. The required amount is added to the skimmer box with the filter valve set on recirculate. 

Do not use excess Lo-Chlor Chlorine Remover because you will not get a Chlorine reading until all the Chlorine Remover has been neutralised.

Note: All of the below chemical additions assume you have a 50,000L pool, if the size of your pool differs from this you will need to recalculate the chemical additions to suit your pool size.

Iron and Manganese Stain Removal:

    • Backwash and rinse the filter.
    • Remove Chlorine and ensure pH is 7.2 to 7.6.
    • Add 1 Kg of Lo-Chlor Multi Stain Remover, by sprinkling around the outside of the pool.
    • Filter as normal.
    • The stain should be gone within twelve hours. If in the unlikely event this does not remove all the stain, repeat with another 1 Kg.
    • When the stain is gone, add 1L of Lo-Chlor Aquaguard CSM. 
    • Add 300g Lo-Chlor Filter Aid & Metal Remover to a bucket and form a slurry, pour the slurry directly into the skimmer box with the filter running and set to filter. Run the filter for 24 hours straight.
    • After 48 hours (or when the pool clears up if it became cloudy) backwash/rinse the filter 3-4 times to ensure the filter aid is completely removed from the filter. 
    • Add another 1L of Lo-Chlor Aquaguard CSM. 
    • Do not superchlorinate for 10 days after adding Aquaguard CSM, or the stain will return immediately.

Copper Stain Removal:

The longer the Copper stain has been there the longer it will take to remove it. Chlorine must be removed in order for the stain removal to be effective.

    • Backwash and rinse the filter.
    • Adjust pH to 6.8 to 7.2.
    • Add 1Kg each of Lo-Chlor CU Later Part-I and Part-II, in fibreglass and painted concrete pools you can sprinkle it directly over the stained area or evenly distribute it around the pool if the staining is widespread. For quartzon, coloured pebble, coloured marble, exposed aggregate finishes or printed vinyl liners do not allow direct contact with the pool surface. In these cases it is recommended that you mix the solutions separately in a bucket of water and disperse evenly around the edges of the pool.
    • Filter for 24 hours straight.
    • Observe the pool each day to determine when the Copper staining has been removed. 
    • It is acceptable to add another 1Kg each of Lo-Chlor CU Later Part-I and Part-II after two days if stain removal is progressing, but not fast enough.
    • When satisfied with Copper removal, add 1L of Lo-Chlor Aquaguard CSM. 
    • Add 300g Lo-Chlor Filter Aid & Metal Remover to a bucket and form a slurry, pour the slurry directly into the skimmer box with the filter running and set to filter. Run the filter for 24 hours straight.
    • After 48 hours (or when the pool clears up if it became cloudy) backwash/rinse the filter 3-4 times to ensure the filter aid is completely removed from the filter. 
    • Add another 1L of Lo-Chlor Aquaguard CSM. 
    • Do not superchlorinate for 10 days after adding Aquaguard CSM, or the stain will return immediately.

Note: If the stain has been there for more than a month for a pebble or marblesheen pool, it will be quicker and as cost effective to empty and acid wash the pool.

Treating Black Spot in Fibreglass pools:

    • Remove Chlorine.
    • Add 4 Kg of Lo-Chlor Spotaway and leave overnight or up to 4 days, if necessary.
    • Then balance water and Superchlorinate.
    • After 24 hours add 1 Litre of Lo-Chlor Aquaguard CSM.
    • Results will be visible within 5-7 days

Note: This treatment removes the visible Black Spot but does not stop the osmosis which caused it. The Black Spots will reappear after three to six months.

Treating Organic Stains:

    • Adjust pH to 7.0 to 7.2 and
    • Add 20 Litres of Liquid Chlorine.
    • Filter as normal for 72 Hours.
    • This should bleach out any organic stains.
    • Severe stains may be helped by sprinkling Lo-Chlor Black Spot 900 Algaecide directly on to them and leaving it overnight.
    • If the stains were caused by dirt it may also be necessary to treat for Iron as well.

Note: For organic stains in Vinyl pools try 1Kg of Lo-Chlor Multi Stain Remover Carefully follow directions as this should never be added directly to a Vinyl pool surface

Treating Scale:

If the scale has been there less than 2 years:

    • Add 1Kg each of Stain & Scale Attack Part I to a bucket of water, stir to dissolve. Once dissolved add 1Kg of Stain & Scale Attack Part II to the same busket and dissolve.
    • Disperse this solution across the entire pool surface, try to cover as great an area as possible. The pool will immediately go cloudy, this should clear up within 10 minutes.
    • Run the filter for 24 hours straight.
    • After 7 days rebalance the pool water. Add 1L of Lo-Chlor Aquaguard CSM at this stage.
    • It may take up to 6 weeks to start seeing improvements.
    • A second dose can be added after 6 weeks if required.
    • Regular treatment with Aquaguard CSM is recommended moving forward.

If the scale has been there longer than 2 years…

The easiest way to remove it, is to empty and acid wash the pool

For fibreglass pools it is possible to add 25 Litres of Hydrochloric Acid directly to the water and leave the pool running as normal for seven to 14 days.

The water is then rebalanced, adding buffer slowly!

Need help?

Our philosophy is simple – a well-balanced pool shouldn’t harm the environment or your health with harsh chemicals. Interested in how we accomplish this with our pool cleaning services in Sydney? Reach out for a consultation, obligation-free! We love sharing our eco-friendly approach to pool cleaning and maintenance.

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