Staining in Pools

What Is That Stain? What Can I Do About It?

These questions are not easily answered. There are multiple types of stains, multiple reasons for stains to appear and sometimes a combination of staining issues can be at play.

This makes the treatment of staining difficult and is often attempted by the owner without crucial knowledge, resulting in unsatisfactory outcomes. You will benefit from leaving the job of treating any staining challenges to a trusted pool professional like Billabong Pool Service & Supply.

When looking at treating staining in pools, keep in mind the following factors:

  • The history of the stains
  • The type of surface.
  • The age of the surface.
  • A chemical history of the pool
  • The length of time the staining has been present
  • The use of chemical such as algaecides
  • The change in type of sanitation.
  • The change in the state of filtration.
  • Importantly the customer’s expectations on what outcome they seek from the treatment.

The following is a brief description of the type of stains we encounter along with a brief description of how they are treated. This blog is not meant to be an instruction manual on how to treat stains; it is more of an introduction to the complexity of the issues and to give you a starting point to address the issues you have been pondering.

Iron Stains

  • Light brown in colour (similar to weak tea)
  • Very common in fibreglass pools.
  • Causes can include the addition of pool salt (especially in a high chlorine and/or pH environment)
  • Extended use of high levels of liquid chlorine
  • High pH is another common factor

Citric or Ascorbic acid may lift this type of stain to sequester them into solution *


  • Dark brown in appearance
  • Can be quite stubborn
  • Will take an extended time to remove (dependent upon the length of time the staining has been present)
  • Caused usually by poor quality pool salt and high pH

Combination of Citric or Ascorbic acid, along with a complex multipurpose acid may lift these stains and sequester them into solution*

Copper Staining (Oxide)

  • Usually black in appearance when copper oxide is formed but can also appear as a green to black stain
  • Causes include inferior chemicals such as simple copper algaecides used in conjunction with calcium-based chlorine
  • Degraded heater elements
  • The long-term use of ionised sanitation systems.
  • The overuse of copper-based products.
  • Not easily removed and are time-dependent (the longer they have been present, the longer and harder they are to remove)

A combination of sodium Carbonate Peroxyhydrate, Sulfamic acid, have proven to be effective. The addition of Tetrahydrate, Oxalic acid and Polybasic organic acid may lift the stains and sequester them into solution*

Osmosis (Blackspots)

  • Not to be confused Blackspot algae
  • Only appears in fibreglass pools
  • Water molecules are so tiny that they actually pass through the gel coat (surface) layer of the pool and combine with the first layer of resin and forms larger molecules, so when the combined molecule tries to pass back through the gel coat layer, it’s too large to fit. This results in damage to the gel coat in the form of osmotic blisters.
  • The water in the molecules turns black
  • Also, a cobalt reaction with the surface can turn these blisters black

The use of Sodium Metabisulfite and ongoing reduction and control of pH level can be a short-term remedy for this challenge (you may have to revisit this problem within 6 months). An extensive repair or full resurface would be a far better option to consider.

Organic Stains

  • Usually introduced by environmental factors
  • Leaves, plant matter, algae, tannins leaching into the pool or overhanging trees.
  • Often mistaken for mineral stains (organic stains will react quicker to stain removal processes)
  • Can be time-dependent (the longer they have been present, the longer and harder they are to remove)

The use of Oxalic acid and Polybasic organic acid may lift the stains and sequester them into solution*

Rust Stains

  • Can be grouped into two categories.
  • Present in pebble pools where steel reinforcing is used and is very close to the surface of the pool and oxidises over a long period of time (mechanical intervention is the only resolution that can be used)
  • Caused by incorrect materials that have been used in the pool, handrails, steps, light fittings, pool toys, etc.
  • Building works carried out around the pool with metal objects being allowed to enter the pool and left sitting on the surface.

The use of Oxalic acid and Polybasic organic acid may lift the stains and sequester them into solution* (depending on the surface type, a mechanical intervention with an abrasive product may be useful)

Calcium Scale

  • Calcium scale can appear as calcium sheen across the surface. As spots or lesions on the surface
  • It can be misdiagnosed as another type of stain, as the calcium scale supplies a bed that attracts the stain.
  • While initially removing the staining. The scale will remain to attract further staining.
  • Causes are many and it is very unlikely that any pool will not have some level of scaling.
  • Main cause is high LSI (Langelier Saturation Index), an index that determines how likely scale will form as sheets across the surface.
  • A combination of high pH, high Total Alkalinity and high Calcium Hardness will produce a high LSI.
  • This will enable Calcium Hydroxide to combine with Carbon Dioxide in solution and form Calcium Carbonate and H20 which plates out as scale on the surface
  • Spots and lesions usually form due to the opposite condition. Very low LSI produces aggressively corrosive water to draw calcium from the substrate.
  • Very difficult to treat and very difficult to prevent.

Combination of sodium Carbonate Peroxyhydrate and Sulfamic acid may be effective. The addition of Tetrahydrate, Oxalic acid and Polybasic organic acid may also be useful in lifting the stains and sequester them into solution*. In the long-term correct water chemistry and some form of pH control.

All of the above treatments require very low chlorine levels 1.0 ppm or less and low pH around 7.0. It is recommended that, in our climate, these treatments are more likely to succeed and cause less disruption during the cooler months.

*You will note that in most of the stain treatments, I talk about the stain being sequestered into solution. This is the most neglected side of stain removal and results in an unsatisfactory outcome in most cases. When the staining substance is in solution, and we have to return the pool to its normal chemistry, there is a real potential for that substance to drop out of solution and plate out on the surface again. We have two choices:

  • Chemically keep the substances in solution. 
  • Chemically remove from solution, or physically remove them through dilution or in extreme cases complete draining of the pool.

Billabong Pool Service & Supply is fully equipped with the chemicals, experience and know-how to resolve your staining challenges. Please come and visit us at one of our three locations at Caloundra, Maroochydore and Noosa so we can to discuss with you your options and develop a realistic plan to help you.

As always, we hope you have found this blog informative and useful.

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