Water Discolouration

What Is Water Discolouration?

Water discolouration is an abnormal change in swimming pool water colour caused by dissolved metals, algae, organic compounds or chemical reactions. Affected water may appear green, brown, yellow, blue-green or dark, depending on the contaminants present and their chemical state.

Discolouration differs from cloudy water, which describes reduced transparency caused by suspended particles or other light-scattering substances. Coloured water can remain transparent, become cloudy or develop both conditions simultaneously.

Water colour is a diagnostic clue, not proof of a particular contaminant. Identifying the cause requires water chemistry testing, visual inspection and an assessment of recent pool maintenance.

Common Pool Water Colours and Possible Causes

The following colour patterns can help narrow down potential causes, but none provides a definitive diagnosis.

Green Water

Green water commonly develops when algae multiply under inadequate sanitisation. Low effective chlorine levels, warm conditions and poor circulation can contribute to algae growth.

Copper contamination is another possibility, particularly when water develops a green tint after chemical treatment. Clear green water may suggest a dissolved contaminant, whereas cloudy green water may indicate suspended material, including algae.

Neither appearance confirms the cause without testing.

Brown or Tea-Coloured Water

Brown water is often associated with iron oxidation or natural organic pigments.

Dissolved ferrous iron (Fe²⁺) can be oxidised by chlorine to ferric iron (Fe³⁺). Under suitable water chemistry conditions, ferric iron forms poorly soluble compounds, including iron hydroxides, that produce yellow, orange or brown suspended particles and deposits.

Tannins released by decomposing leaves, bark and vegetation can also create a transparent tea-coloured appearance.

Yellow Water

Yellow discolouration may result from low concentrations of iron compounds, dissolved organic pigments or fine suspended material.

It should be distinguished from mustard algae, which typically forms yellowish or brownish deposits on pool surfaces rather than uniformly colouring the water.

Blue-Green Water

Copper-containing compounds can produce blue-green discolouration or surface staining under certain chemical conditions.

Potential copper sources include copper-based algaecides, mineral treatment systems and corroding heat exchangers.

However, blue or turquoise water is not automatically abnormal. Pool depth, lighting, water clarity and interior surface colour can create a naturally blue appearance.

Black or Dark Water

Dark discolouration may be associated with oxidised manganese compounds or substantial organic contamination.

Manganese can form dark-coloured precipitates when its oxidation state and solubility change.

Black algae, by contrast, generally appears as persistent dark patches attached to pool surfaces rather than turning the entire water body black.

Why Does Swimming Pool Water Change Colour?

Dissolved Metals and Oxidation

Iron, copper and manganese can enter swimming pools through source water, including bore water and groundwater, as well as treatment chemicals and equipment corrosion.

Some metals remain dissolved until changes in oxidation conditions or pH alter their chemical form. They may then produce coloured compounds, suspended particles or surface deposits.

Chlorination can make existing metal contamination more visible by promoting oxidation. The resulting colour change does not necessarily mean the chlorine product introduced the metal.

Algae Growth and Inadequate Sanitisation

Algae can develop when sanitiser effectiveness is insufficient to control biological growth.

Free chlorine concentration, pH, cyanuric acid (CYA), sunlight exposure and organic contamination all influence pool sanitisation.

CYA protects chlorine from rapid degradation by sunlight but also changes chlorine's disinfection behaviour. Maintaining an appropriate relationship between stabiliser and sanitiser levels is therefore important in outdoor pools.

Organic Matter and Natural Pigments

Leaves, bark and other vegetation release water-soluble organic compounds as they decompose.

Tannins and related pigments can give the pool a yellow or brown appearance even when the water remains relatively transparent.

Australian pools surrounded by eucalyptus trees and other vegetation may be particularly exposed after strong winds, storms or heavy leaf fall.

Water Chemistry and Equipment Corrosion

Water chemistry influences both metal solubility and equipment durability.

Low pH and chemically aggressive water can accelerate corrosion of susceptible metal components, including copper heat exchangers and fittings, releasing metal ions into the circulation system.

Conversely, elevated pH can reduce the solubility of certain iron, copper and manganese compounds, increasing the risk of precipitation, water discolouration and surface staining.

Maintaining water balance appropriate to the pool's construction and equipment helps limit these problems.

How to Diagnose Discoloured Pool Water

A systematic diagnosis helps distinguish biological contamination from metals and organic pigments.

1. Test the Core Water Chemistry

Measure free chlorine, total chlorine, pH and total alkalinity. In outdoor pools using chlorine stabiliser, also test cyanuric acid (CYA), which influences chlorine stability and sanitising effectiveness.

Australian water quality guidance provides useful reference values. For example, Queensland Health's Water Quality Guidelines for Public Aquatic Facilities (2019) specifies the following criteria for chlorine-treated public pools:

Source: Queensland Health, Appendix 2, Table A2.1.

These criteria apply to the public aquatic facilities covered by the Queensland guidance. Private pool owners should follow applicable local requirements, relevant manufacturer instructions and appropriate water treatment advice.

NSW Health also publishes water quality requirements for public swimming pools and spa pools, while Australian Standard AS 3633-1989 addresses water quality in private swimming pools.

2. Check for Metal Contamination

Test for iron, copper and, where relevant, manganese.

Be aware that some tests measure only certain dissolved forms of a metal. Sequestered metals or suspended precipitates may not be fully represented in the result.

If initial readings do not explain the discolouration, a pool water specialist may recommend more comprehensive analysis.

3. Inspect Pool Surfaces and Equipment

Look for algae deposits, metal staining, corrosion and accumulated debris.

Check susceptible components such as heater heat exchangers, fittings and relevant plumbing. Surface staining may indicate a related metal problem, although staining and water discolouration are not the same condition.

4. Review Recent Water and Chemical Additions

Determine whether the colour change followed:

• Filling or topping up with bore or other source water.

• Chlorination or pH adjustment.

• The use of copper-containing pool products.

• Heavy rainfall, runoff or organic debris.

• Equipment deterioration or maintenance.

A clear timeline can help identify the likely source before treatment begins.

How to Treat Water Discolouration

Treatment should address the confirmed cause rather than the water colour alone.

Algae-Related Discolouration

When algae are identified, restore suitable sanitiser conditions, brush affected surfaces and maintain effective filtration and circulation.

The treatment procedure should account for the pool's CYA concentration, sanitiser system, surface materials and product instructions.

Clean or backwash filtration equipment as required. Avoid returning the pool to use until water clarity and chemistry meet applicable safety requirements.

Metal-Related Discolouration

For confirmed iron, copper or manganese contamination, assess the metal source before applying treatment.

A compatible metal sequestrant may help keep metal ions in a controlled dissolved form and reduce precipitation or staining. However, sequestration does not physically remove the metal from the water.

Actual metal reduction may require suitable metal-removal media, specialised water treatment or partial replacement with tested low-metal water. Ordinary pool filtration generally cannot remove fully dissolved metal ions without an additional treatment process.

Because oxidation and pH changes can worsen some metal-related conditions, avoid indiscriminate shock chlorination or aggressive chemical adjustments.

Organic Discolouration

Where tannins or other organic pigments are responsible, remove the underlying vegetation and accumulated debris.

Maintain appropriate sanitisation and circulation while monitoring whether the colour diminishes. Persistent contamination may require additional treatment suited to the pool's filtration system and water chemistry.

Correct the Underlying Source

If discolouration results from corroding equipment, contaminated top-up water or repeated chemical additions, the source must be addressed to prevent recurrence.

Partial water replacement may be considered when appropriate, but replacement water should be tested first. Draining must also account for groundwater pressure, pool construction and local water restrictions.

How to Prevent Water Discolouration

Consistent maintenance reduces the likelihood of biological growth, metal contamination and organic staining.

• Test water regularly: Monitor pH, sanitiser, alkalinity and CYA where applicable.

• Check source water: Test bore water and other unfamiliar supplies for metals before pool filling or top-ups.

• Maintain water balance: Follow appropriate operating recommendations for the pool surface and equipment.

• Inspect equipment: Check susceptible metal components for corrosion and deterioration.

• Manage organic debris: Remove leaves, bark and other vegetation before prolonged decomposition.

• Maintain filtration: Keep filters and circulation equipment operating according to manufacturer requirements.

During Australian summer conditions, high UV exposure can increase chlorine loss, while warm water and organic contamination can increase sanitiser demand. Testing requirements may also change after heavy rainfall, major water additions or periods of intensive pool use.

Technical References

1. Queensland Health (2019). Water Quality Guidelines for Public Aquatic Facilities. Appendix 2, Table A2.1. Official Guidelines

2. NSW Health (2026). NSW Guidelines for Managing Water Quality in Public Swimming Pools and Spa Pools. Published 12 August 2026. Official Guidelines

3. Standards Australia (1989). AS 3633-1989: Private Swimming Pools — Water Quality. Australian Standard addressing water quality requirements for private swimming pools.

4. Pool & Hot Tub Alliance (PHTA). Metals and Metal Staining. Technical guidance on metal contamination, discolouration, sequestration and staining.