Pool stains should not be diagnosed by colour alone. A brown mark may be organic debris or iron, a black spot may be biological growth or metal staining, and a white mark may be calcium scale or a change in the surface itself. Use appearance, texture, location, pool history and a small surface-appropriate check together before choosing treatment.
Which type of pool stain does it look like?
Organic staining
Appearance: brown, tea-coloured, olive, green-brown or irregular dark marks.
Texture: usually flat; fresh residue may brush or smear.
Common location: floors, steps, shaded areas and low-flow zones where leaves, pollen or other debris collect.
Best clue: often follows debris loading, storms or periods of poor sanitation.
Metal staining
Appearance: yellow-brown, orange, rust-coloured, grey, blue-green or dark fixed marks.
Texture: normally flat rather than crusty.
Common location: below returns, near ladders or handrails, around metal fittings or along refill-water paths.
Best clue: the mark resists brushing and may follow a metal source or water path.
Black spotting / possible black algae
Appearance: small black, blue-black or very dark green spots, sometimes grouped in clusters.
Texture: may remain fixed or feel slightly rough where growth is established in a porous finish.
Common location: plaster, pebble and other porous surfaces, especially shaded or poorly brushed areas.
Best clue: persistent dark heads in porous material can support a black algae diagnosis, but do not confirm it by themselves.
Calcium scale
Appearance: white, grey, pale tan or chalky mineral build-up.
Texture: rough, raised, crusty or gritty.
Common location: waterline, tile, spillovers, fittings and other evaporation-prone areas.
Best clue: a rough or raised deposit is more consistent with scale than a flat colour change.
Safe first checks for common pool stains
| Type | Best identifying clue | Safe first check | Stop DIY when |
|---|---|---|---|
| Organic | Often follows leaves, pollen, debris or a period of poor sanitation and may improve with suitable brushing. | Remove loose debris and brush a small area with equipment suitable for the pool finish. Improvement after brushing and normal sanitiser control supports an organic cause. | The finish is unknown, sanitiser is already high, or the mark is rough, raised or clearly associated with metal fittings. |
| Metal | Usually flat, difficult to brush away and sometimes follows returns, fittings, heaters or refill-water paths. | On a small suitable area, noticeable lightening with vitamin C / ascorbic acid supports a metal stain, particularly iron, but does not identify the specific metal. | The source has not been investigated. Chlorine and other oxidisers can cause some dissolved metals, particularly iron, to oxidise and precipitate, making staining more visible. |
| Black spotting / possible black algae | Persistent dark heads in a porous finish, particularly in shaded or difficult-to-brush areas. | Brush one small area using a brush suitable for the finish. Use the response together with the surface type, sanitation history and location. | The pool is vinyl, fibreglass, painted or has an ageing finish and you are considering concentrated chlorine contact or aggressive brushing intended for another surface type. |
| Calcium scale | A rough, raised, chalky or crusted deposit rather than a flat colour change. | Check texture and review the pool's calcium carbonate saturation balance using the relevant water-balance factors, commonly expressed as LSI or CSI. | The finish type or water balance is unknown. Do not use an acid wash simply to identify an unexplained deposit. |
A safe identification sequence before treatment
Note the shape, colour and location, whether the mark follows plumbing or fittings, and whether it appeared after debris, top-up water or chemistry changes.
Flat staining points in a different direction from a raised mineral deposit. Texture is especially useful when separating calcium scale from discolouration.
Start with brushing or another controlled test compatible with the pool finish. Do not layer several chemical tests onto the same area.
Combine the response with the water history, surface type and stain location before choosing a treatment.
Match the treatment to the likely cause
Organic staining
Remove the debris source, restore normal sanitiser control and brush with equipment suitable for the finish. If staining repeatedly forms in the same area, check circulation and cleaning coverage.
Metal staining
Investigate the source before treating the whole pool. Possible contributors include fill water, heater components, metal fittings, corrosive water conditions and previous metal-containing pool products.
Possible black algae
If the location, porous surface and brushing response support black algae rather than a metal or mineral stain, use a treatment appropriate to that pool finish instead of treating every dark mark as algae.
See how to distinguish black algae from other algae and dark pool marksCalcium scale
Correct scale-forming water conditions before choosing a removal method. Acid treatment reacts with and removes material from the outer layer of cementitious pool finishes, so unnecessary or excessive treatment can accelerate surface wear.
Check why the stain formed
- Organic staining: look for recurring debris loading, poor brushing coverage, low circulation or periods of inadequate sanitiser control.
- Metal staining: investigate fill water, heater condition, metal fittings, corrosion and previous metal-containing products.
- Possible black algae: check porous or damaged areas, shaded zones, sanitation history and areas that are difficult to brush effectively.
- Calcium scale: assess the pool's calcium carbonate saturation balance, including the relevant factors used in LSI or CSI rather than relying on pH alone.
When to stop DIY stain treatment
- The visual clues and test response do not point to one likely cause.
- A dark mark follows metal fittings or repeatedly returns after treatment.
- The surface is painted, vinyl, fibreglass, soft, etched or already worn.
- You are considering an acid wash mainly because ordinary brushing failed.
- Scale is widespread rather than limited to one small waterline area.
- The pool has both staining and unstable water chemistry.
- You have already tried several chemical treatments and the mark is becoming darker or changing colour.
Pool stain identification FAQ
Can I identify a pool stain by colour alone?
No. Brown can be organic staining or iron, dark staining can be biological or metal-related, and dirty calcium scale may look grey or tan. Use colour together with texture, location, pool history and a small surface-appropriate check.
How can I tell an organic stain from a metal stain?
Organic stains are more likely after leaves, debris or sanitation problems and may improve with suitable brushing and normal sanitiser control. Metal stains are usually flat, resist brushing and may follow returns, fittings or refill patterns. Noticeable lightening with vitamin C supports a metal stain, particularly iron, but does not identify the specific metal.
Does every black pool spot mean black algae?
No. Dark marks can also come from metals, trapped contamination, mineral deposits or changes in the pool finish. Persistent dark heads in a porous surface can support a black algae diagnosis, but colour alone is not enough.
How do I tell calcium scale from a flat stain?
Texture is the strongest early clue. Calcium scale normally feels rough, raised, chalky or crusted, while most organic and metal stains are primarily changes in surface colour. Water testing and the pool's calcium carbonate saturation balance, commonly assessed with LSI or CSI, can help confirm whether scale-forming conditions are present.
Should I acid-wash a pool if the stain is severe?
Not as a first diagnostic step. Acid treatment reacts with and removes material from the outer layer of cementitious pool finishes. Identify the likely stain, confirm the finish and assess the water conditions before deciding whether acid washing is appropriate.