1Read the colour (as a clue only)
| What you see | Likely cause |
|---|---|
| Green, brown or black under leaves, or where debris has been sitting | Organic: leaves, gum nuts, algae |
| Red-brown, orange-brown, rust, sometimes yellow | Iron |
| Blue-green; can also look grey or black | Copper |
| Dark brown-black to purple-black | Manganese |
| White or grey, rough and gritty like sandpaper, often raised | Calcium scale, not a stain |
Black algae in plaster or pebble pools is easily mistaken for a dark metal stain. Scale can look like one too, so feel the surface.
2Chlorine spot test
Hold a chlorine tablet or some granules directly on the stain for about one minute to a few minutes. Don't leave it longer than needed.
Not on vinyl, fibreglass or painted surfaces. Concentrated chlorine can bleach them and cause permanent damage.
It's organic. Raise the chlorine and brush the stain, with a brush that won't scratch the surface, until it's gone. Follow the product label.
It isn't organic. Do the vitamin C test.
3Vitamin C spot test
Crush some vitamin C (ascorbic acid) tablets into a sock and hold it against the stain for 30 seconds to a minute.
| What you see | What it means | What to do |
|---|---|---|
| Stain lightens or fades | Metal, most likely iron | Find the source, then treat with a metal stain remover such as AquaGUARD Citoxal Metal Stain Release (follow the label) and sequester (below) |
| Stain darkens instead | Suspect copper; vitamin C is largely ineffective on it | Confirm with a stain ID kit or a LaMotte Insta-Test copper strip. Copper stains are normally removed with sulfamic acid; follow the label |
| No change from either test | Possibly not a stain | See When it isn't a stain |
Test the pool and top-up water to identify the metal and whether the top-up is adding it. A clear water test doesn't rule out a metal stain.
Perth: iron from bore-water top-ups
Iron staining from garden bores is common in Perth: dissolved iron reacts with air once it's pumped to the surface. Bore staining can also be manganese, so a dark bore stain isn't necessarily copper.
Healthy WA prefers scheme drinking water for filling a pool, and says bore water should never be used to fill one unless it has been tested and treated for that use. It can stain, and iron-rich bore water increases the chemicals needed. Have a bore lab-tested (NATA-accredited) before you rely on it for filling.
Check the address on DWER's iron staining risk map. On the Perth Groundwater Map, search the property and open the iron staining risk tab (iron and manganese, 2004 atlas data). It doesn't cover every risk area, so ask whether neighbouring bores stain too.
Where copper comes from
Look for:
- Copper-based algaecides
- Ionisers and other mineral systems
- Low pH eating into a heater's copper heat exchanger or old copper plumbing. Corrosive (negative LSI) water attacks metal fittings in general.
After a vitamin C treatment: sequestrant before chlorine
Ascorbic acid doesn't remove iron; it dissolves the stain back into the water. Chlorine and ascorbic acid destroy each other, so free chlorine needs to be below 1 mg/L (ppm) while the treatment works.
Add a metal sequestrant such as AquaGUARD Avert before you re-chlorinate, or the iron is likely to come back out of solution and stain again. Sequestrants slowly break down, so top up regularly if the water source has metals in it.
Follow the label for every product in this sequence.
When it isn't a stain
- Calcium scale. White or grey, rough and gritty. It forms when the water is over-saturated with calcium carbonate (positive LSI). Correct pH, alkalinity and calcium hardness to stop more forming; lift existing scale with a scale remover such as AquaGUARD Eliminate (follow the label) or acid treatment. Check the LSI calculator, and see calcium scale and the LSI.
- Stain keeps coming back. The metal is still in the water or the top-up supply. Sequester it and find the source.
Final step
Find the metal's source before you treat: check the address on the risk map, test the top-up water, and check the heater and algaecide history. Then treat, sequester and re-chlorinate slowly, following each product's label.
Based on
- WA Health, Code of Practice for aquatic facilities (Appendix 7, water balance)
- Healthy WA, Swimming pools and spas and Bore water
- WA DWER, Water Quality Protection Note 55: Swimming pools
- Be Groundwater Wise, The right advice for garden bores and DWER Iron Staining Risk (DWER-032)
- Australian state health guidelines for public swimming pools (water balance)
- Pool chemical manufacturers' and trade guides to stain identification and treatment
- International industry practice (ascorbic acid treatment)
Checked against these and other manufacturer and trade sources, October 2026. Always follow 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.