1Brush it
Small dark spots, often in corners, on steps, in grout and on porous surfaces such as pebblecrete. It roots into the surface, so brushing alone won’t shift it.
Looks like sand, pollen or fine yellow-brown dust. It brushes away very easily but comes back to the same place, often within a day.
2Is it algae at all?
- Sand or dust? As a rough field test, brushed algae tends to hang in a cloud, while sand settles straight back. If it’s filter media, see the media filter field guide.
- A stain? Metal stains usually lighten under vitamin C (ascorbic acid) or a stain remover, though copper can darken instead. Black spot doesn’t lift. See pool stain identification.
3What’s the surface?
Black spot needs a porous surface, so it is rare in vinyl and fibreglass. Some manufacturers note that black spots in fibreglass are usually cobalt stains from osmosis of the gel coat, often with blisters at the spot, not algae.
| Surface | Brush |
|---|---|
| Vinyl and fibreglass | Soft nylon only. Anything stiffer scratches the finish. |
| Concrete, render, plaster, tiled | A stiff nylon brush. Glazed and glass tiles scratch easily, so stay soft on those. |
| Pebblecrete | Stiff nylon for routine brushing. On the spots only, the stiffest brush the finish maker’s care guide allows. |
Some black spot product labels call for a stiff brush on the spots. Use the stiffest brush that won’t scratch the finish, and check the interior finish maker’s care guide before you go harder than nylon. A scratched finish is permanent; the algae isn’t.
Treating black spot
Treat with no swimmers in the pool. Algaecides must be APVMA-registered: AquaGUARD Clear Control is APVMA No. 52194 and AquaGUARD Clear Guard is APVMA No. 63543.
- Brush hard. Without brushing, even the right chemical struggles to work.
- Raise the sanitiser, for example with liquid chlorine (AquaGUARD AquaFresh 1250). Follow the label.
- Apply the black spot product to the spots. Granular products are typically trichlor placed directly onto the spots; their labels limit them to pebblecrete, white plaster and tiled pools, because they can bleach other finishes. Liquid copper or quat black spot algaecides are the alternative, including where granules can’t be used: AquaGUARD Clear Control is a copper algaecide and AquaGUARD Clear Guard a non-copper (quat) algaecide. Use one only if its label covers black spot, and read the copper cautions below first.
- Wait, then brush again. Labels give different wait times, and some direct turning the filter off during treatment. Follow the label on your product.
- Remove the dead algae. Vacuum to waste, or vacuum through the filter and then backwash or clean it. In WA, domestic pool water can’t go to sewer: see getting rid of the water in our green pool guide.
Using a liquid product? Some liquid black spot product labels say not to superchlorinate for 48 hours before and after adding it. Follow the label.
Set expectations. Some product makers say black spot eight to twelve months old can take up to a month to clear, and about half of pools need a second dose. Follow the label.
Watch the stabiliser. Trichlor adds stabiliser with every dose, and some granular product directions say not to use the product once stabiliser reaches 50 mg/L (ppm). Stabilised chlorine can make a shock less effective, so test stabiliser before repeated trichlor treatments, or bring a sample in for a Spin Touch analysis. See stabiliser too high.
Treating mustard algae
Mustard re-infests the pool from swimwear, automatic cleaners, nets, poles, brushes and toys that were in the water with it. While you treat the pool:
- Clean equipment and toys in a liquid chlorine solution as the product directions say, or leave them in the pool while it’s shocked.
- Wash swimwear before it goes back in the pool.
- Clean the filter thoroughly, because mustard can survive in it. See cartridge filter cleaning.
- Brush everywhere it clings, including behind light fittings, ladders and wall fittings.
Copper algaecides: check first
Copper algaecides add copper to the water, and copper can stain. Some copper algaecide labels say not to use them in pools with ionisers, and some also rule out biguanide pools. Check the label before you dose.
Keeping them away
- Chlorine matched to stabiliser. Stabiliser reduces chlorine’s disinfecting power, so stabilised pools need a higher minimum free chlorine. The WA minimums are in green pool recovery.
- Circulation. Dead spots and poor circulation let both algae take hold.
- Brush weekly, especially dead spots and low-flow areas.
When it isn’t black spot or mustard
- Green water or a slimy film: see green pool recovery.
- Marks that don’t brush or lift: see pool stain identification.
- Grit at the returns: see the media filter field guide.
Final step
Never mix chlorine products with each other or with acids. Add the product to water, never water to the product. Use every product as its label and SDS direct. See WorkSafe WA’s pool and spa chemicals guidance.
Based on
- WorkSafe WA, pool and spa chemicals guidance
- Australian state health guidelines for public aquatic facilities
- Black spot and algaecide product labels, directions and safety data sheets
- Pool chemical manufacturers’ and pool builders’ guides to black spot and mustard algae
- International industry practice (mustard algae)
Checked against these and other manufacturer and trade sources, October 2026. Always follow the manufacturer’s manual and the product label.
General information for pool and spa trade professionals. Every pool is different. This guide does not replace the product label, the SDS, the equipment manual or advice from a licensed tradesperson, and figures quoted from the WA Code of Practice apply to public aquatic facilities. To the extent permitted by law, AGent Sales & Services Pty Ltd accepts no liability for loss or damage arising from reliance on it. Website terms.