Stain Removal & Prevention

All you need to know about metal and organic stains and most importantly – how to remove them...

Stains are a common occurrence in pools but nothing about stains is simple. There is no simple 1,2,3 for stains. What works well on one stain may not be nearly so 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.

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).

Stains can be the size of a hair clip or to where the whole pool surface is stained.

It is rare to find a pool that does not have stains of some description and it is a case of what the pool owner sees as beyond acceptable that determines if or when you will be called in.

What causes stains?

Stains are primarily 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 hosing. 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 leaches 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 combines with a low pH. The use of Trichlor Tablets which have both a high Chlorine content and a low pH need 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.
  • Use of bore water. See separate information sheet on bore water.

Look for the cause:

There is nothing worse than spending time and effort in removing a stain only to have it reappear after three weeks. This is a very common scenario when using something like Citric Acid. The initial dose works well but Chlorine slowly breaks down the Iron/Citric Acid complex and the stain reappears.

Remember to look for the cause. Salt is the biggest cause of stains in pools so it is important the pool owner realises that stains are an ongoing problem in salt pools.

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 or if it is possible to remove the stain completely.

The first step in testing the stain is to discuss with the pool owner what they have added recently which may have caused the stain. Usually a pool owner will have some clue as to how the stain formed.

  • Was it after adding salt?
  • A Copper based Algaecide?
  • Do they have an Ioniser?
  • Do they have a leaf problem and leave the leaves sit in the pool for weeks?
  • Has there been a storm and dirt 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:
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:
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



The Test:

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

Place a handful of Lo-Chlor Multi Stain Remover in a skimmer sock and use the pool broom 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 are removed instantly leaving a clear stripe.

A Manganese stain is removed within two minutes.

Black Spot in fibreglass pools is also removed in two minutes.

Finally a Copper or Silver stain is removed within five minutes. 
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 (how long it’s been there) the scale is.

Testing specifically for Copper:

A handful of Lo-Chlor Nail the Scale is put into a skimmer sock, and using the pool broom; the test 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 is absorbed so far into the surface that an acid wash may only remove the surface stain and the stain may re-appear 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 test repeated. If there is no effect within a minute let the sock sit in one place for three minutes.

Organic stains are removed within three minutes.

Stain Removal:

Having determined what the stain is you can now set about its removal.

Removing the stain from the walls of the pool is only half the battle. The stain is still there but in solution. It is very common to remove a stain only to have it reappear after three weeks or two months.

Before any stain removal the Chlorine level in the pool must not be more than 3.0 PPM. Chlorine will neutralise the stain removers and make them less effective.

In the middle of summer it is sufficient to turn of the salt chlorinator for two days beforehand or not add any Chlorine for two days.

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.

Treating Iron and Manganese Stains:

  • 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 300mls of Lo-Chlor Metal Solution
  • Run the filter for 12 – 24 hours
  • Backwash (for Sand Filters) or clean cartridges thoroughly
  • Repeat this treatment of Lo-Chlor Metal Solution every 3 days until the bottle is empty
  • Do not superchlorinate for 10 days after adding Metal solution, or the stain will return immediately.
  • If the stain was severe it may be necessary, after stain removal, to adjust pH to 7.8 to 8.0 and add 1 Litre Lo-Chlor Maxi Floc Plus.
  • Making the pool alkaline and then floccing will remove about 70% of the dissolved metals.
  • Leave overnight and then vacuum to waste.
  • Balance the water and then add 1 Litre of Lo-Chlor Metal Solution.
  • For a salt water pool this treatment may need to be repeated every six months!

The common Metal Stain…

Iron / Manganese in a Fibreglass pool

Treating Copper Stains:

The longer the Copper stain has been there the longer it will take to remove it. Chlorine level must not be more than 3.0 PPM.

  • Adjust pH to 6.8 to 7.2.
  • Add 2 Kg of Lo-Chlor Nail the Scale, by sprinkling it around the edge of the pool.
  • Filter as normal.
  • Observe the pool each day to determine when Copper removal is complete.
  • It is acceptable to add another 2 Kg of Lo-Chlor Nail the Scale after two days if stain removal is progressing, but not fast enough.
  • When satisfied by Copper removal, adjust pH to 8.0 with Soda Ash.
  • Add 1 Litre Lo-Chlor Maxi Floc Plus and turn off the pump.
  • Leave off overnight and next day vacuum to waste.
  • This is to remove the excess Copper, now in solution, from the treatment.
  • Balance the water and add 1 Litre of Lo-Chlor Metal Solution.

Note 1: If the stain has been there more than a month, for a vinyl pool, the only way is to add 500mls Lo-Chlor Metal Solutionevery week for 6 weeks. The stain will take at least 12 weeks to fade. It may not disappear completely and leave a shadow or outline of the stain.

Note 2: 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.

The copper stain…

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 Metal Solution.
  • Results will be visible within 5-7 days


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

Organic Stains

Treating Scale:

If the scale has been there less than 2 years…

  • Add 2 Kg of Lo-Chlor Nail the Scale, by sprinkling around the pool.
  • It may take up to 6 weeks to remove the scale.
  • A second dose can be added after 2 weeks if required.

Note: Nail the Scale will remove some Chlorine and lower TA slightly. Remember to monitor these.

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!

Remember - Look for the cause!

There is nothing worse than spending time and effort in removing a stain only to have it reappear after three weeks. This is a very common scenario when using something like Lo-Chlor Citra-Clean.

The initial dose works well but Chlorine slowly breaks down the Iron/Citric Acid complex and the stain reappears.

Salt is the biggest cause of stains in pools so it is important the pool owner realises that stains are an ongoing problem in salt pools.