Purple colouration in swimming pools is almost never caused by algae. It is nearly always copper cyanurate (also called purple cyanurate or “purple haze”), a chemical mineral compound forming when elevated cyanuric acid (CYA above 100 ppm) reacts with dissolved non-chelated copper in pool water, precipitating as purple or amethyst-coloured crystalline dust settling on walls, floor, and pool surfaces. True purple algae does not exist as a common pool contaminant. The underlying chemistry: non-chelated copper ions bond with cyanuric acid molecules forming insoluble copper-cyanurate crystals (Na₂Cu(C₃H₂N₃O₃)₄·6H₂O) that settle as purple grit or dust on pool surfaces, most visible in corners, along waterlines, and on pool steps.
This guide covers copper cyanurate chemistry and how it forms, copper sources in pool water (algaecides, ionisers, corroding heaters, fill water), cyanuric acid accumulation from stabilised chlorine products, identification versus true algae and other staining, treatment protocol (CYA reduction via partial drain, sequestrant, metal removal), prevention, and natural pool absence of both cyanuric acid and copper treatment chemicals that cause this condition.
Key Takeaways
- Purple = copper cyanurate, not algae – chemical mineral compound, not organism
- Requires two conditions simultaneously: CYA above 100 ppm AND non-chelated copper in water
- CYA source: Dichlor shock, trichlor tablets/sticks release CYA with every use, accumulating over season
- Copper source: Non-chelated copper algaecides, pool ionisers, corroding heat exchangers, fill water
- Cold water accelerates formation – most common at spring pool opening after copper algaecide added at closing
- Treatment: partial drain to lower CYA – no chemical treatment removes cyanurate without dilution
- Prevention: CYA 30-50 ppm – never allow to exceed 80 ppm, use chelated copper algaecides only
- Natural pools: condition impossible – no CYA, no copper treatment chemicals
Natural Pools Don't Have an Algae Problem
What “Purple Algae” Actually Is
True Purple Algae Does Not Exist in Pools
Purple algae as a common residential pool contaminant does not exist. Rhodophyta (true red/purple algae) is a marine or specialist freshwater organism incompatible with chlorinated residential pool conditions. It dies within hours in chlorinated water.
Any purple discolouration in a swimming pool is almost always one of two things: copper cyanurate (far more common) or manganese staining (rare). Both are chemical mineral problems, not biological organisms.
Copper Cyanurate (Purple Cyanurate)
What it is: Copper cyanurate is a purely chemical reaction and not an organism, algae, bacteria, or biological problem. It forms when cyanuric acid at elevated concentration reacts with non-chelated dissolved copper, producing insoluble crystalline compound that precipitates out of solution depositing on pool surfaces.
Appearance: Purple, violet, or amethyst-coloured dust or grit settling on pool surfaces. Fine crystalline texture resembling purple powder or fine sand. Colour can range from pale lavender to deep violet depending on concentration.
Location: Settles in corners and low points, along waterline tiles, on pool floor, on steps, visible in pool cleaner bags. It does not float, it sinks as crystals form.
Water clarity: Pool water often remains clear or only slightly tinted. This is a key distinguishing characteristic. Algae problems cloud water. Copper cyanurate settles as a deposit while leaving water relatively clear above.
Chemistry: Non-chelated copper (Cu²⁺ free ions) bonds with cyanuric acid molecules forming the insoluble compound Na₂Cu(C₃H₂N₃O₃)₄·6H₂O. This precipitation removes copper from solution and creates visible purple crystalline deposits.
Two Required Conditions
Both conditions must be present simultaneously for copper cyanurate to form.
Condition 1: High Cyanuric Acid (CYA above 100 ppm)
What CYA is: Cyanuric acid (also called chlorine stabiliser or conditioner) added to pools to protect chlorine from UV degradation. Without CYA, outdoor pool chlorine degrades 75-90% within 2 hours of direct sunlight. Appropriate CYA level 30-50 ppm.
How CYA accumulates: CYA does not degrade, does not backwash out significantly, and builds up with every addition of stabilised chlorine products.
Trichlor tablets and sticks: Most common cause of CYA accumulation. Every trichlor tablet releases approximately 50% chlorine and 50% CYA by weight. Using trichlor tablets as primary chlorination through full season raises CYA 30-50+ ppm per season in most pools.
Dichlor shock: Releases approximately 56% chlorine and 44% CYA. Using dichlor shock weekly through the season adds substantial CYA.
Accumulation timeline: Pool starting at 30 ppm CYA using only trichlor tablets can reach 80-100+ ppm within 8-12 weeks of summer. No natural degradation mechanism removes CYA, only dilutes it (rain overflow, partial drain, splash-out).
Effect on chlorine: CYA above 80-100 ppm dramatically reduces chlorine effectiveness. Chlorine “locked” means tests at adequate free chlorine level (3 ppm) but biologically inactive. Algae, bacteria, and other organisms exploit this gap.
Effect on copper cyanurate formation: Above 100 ppm, CYA concentration sufficient to react with any available copper forming insoluble compound.
Condition 2: Non-Chelated Copper in Water
Non-chelated copper: Free dissolved copper ions (Cu²⁺) available to react chemically. Distinguished from chelated copper (copper ions bound to chelating agent, stabilised, protected from reacting with other pool chemicals).
Sources of non-chelated copper in pools:
Low-quality copper algaecides: Inexpensive copper algaecides (often sold at large supermarkets or home stores) contain non-chelated copper sulfate. Added as closing algaecide in autumn, the non-chelated copper remains in water over winter. When CYA is also elevated, purple cyanurate forms, typically discovered when the pool is opened in spring.
Pool mineral ionisers: Systems using copper-silver ionisation (PoolRx, Nature2, similar products) release copper ions continuously. Without careful chemistry management, copper builds up. Overdosed ioniser or unit sized for larger pools than installed introduces excess copper.
Corroding heat exchangers: Pool heaters with cupronickel or copper heat exchanger tubes corrode when water pH drops below 7.2 or when water is otherwise corrosive. Dissolved copper released into water. Low pH pools with heaters commonly develop copper contamination over time.
Copper fill water: Some municipal and private well water sources contain elevated dissolved copper from copper supply pipes. Common in older properties with copper plumbing and acidic water.
Chelated copper algaecides (safer): High-quality copper algaecides use chelated copper (EDTA-chelated or similar) which does not react with CYA at pool-relevant concentrations. Using chelated copper algaecide at appropriate dose generally does not cause copper cyanurate formation even with moderate CYA levels.
Why Cold Water Accelerates Formation
Copper cyanurate formation increases at lower water temperatures. Temperature affects solubility of compounds in water: as water cools, compounds that remain dissolved at higher temperatures precipitate out.
Spring opening pattern: Most common discovery scenario is that the pool operator adds copper algaecide (particularly non-chelated) at autumn closing, over winter CYA remains elevated from summer trichlor use, cold winter water promotes crystallisation, pool opened in spring to purple deposits.
Warm water formation: Also possible in warm water when concentrations are sufficiently high, but less common. Rescildo (Pool Spa News documented case) found copper cyanurate in July pool with warm water, very high CYA and copper concentrations.
Identification
Copper Cyanurate vs Algae
Copper cyanurate: Purple crystalline powder or dust. Settles on surfaces. Water remains relatively clear. Does not brush off into clouds of particles (brushing redistributes dust, does not remove it). No odour. Does not respond to shock treatment and shocking may even worsen the discolouration (chlorine oxidises remaining dissolved copper to copper oxide, darkening deposits).
Algae: Green, yellow, black, or reddish-pink biological growth. Clouds water (except mustard algae which clings to surfaces without clouding). Has an earthy or musty smell. Responds (eventually) to shock treatment. Slimy texture in most types.
Identification Test
Vitamin C test: Place vitamin C tablet (ascorbic acid) directly on a purple deposit. If deposit lightens or dissolves within 2-3 minutes, copper-based mineral staining is confirmed. If there’s no reaction, then there’s organic growth (possible algae or bacteria, though unlikely to be purple).
Acid test: Drop diluted muriatic acid (10:1 water:acid) on deposit. Copper mineral deposits may react (colour change) while organic deposits show no reaction.
Water clarity check: If pool water remains clear or only faintly tinted despite visible purple deposits, it strongly suggests mineral staining, not algae (again, algae clouds water).
Manganese Staining (Rare Alternative)
Appearance: Purple-black to dark brown-purple staining. Firmly adhered to surfaces.
Cause: Dissolved manganese in fill water oxidised by chlorine, depositing as manganese oxide on pool surfaces.
Test: Manganese staining does not respond to vitamin C. Responds to specific manganese stain remover or reducing acid treatment.
Prevalence: Much rarer than copper cyanurate. Manganese in fill water is uncommon outside specific geological regions.
Treatment Protocol
Important: Do Not Shock Before Treating
Adding chlorine shock to water containing elevated dissolved copper causes additional reactions. Calcium hypochlorite shock can produce black cupric oxide (CuO) deposits in addition to existing purple staining. Address copper and CYA before shocking.
Step 1: Test Water Chemistry
Test and record: free chlorine, pH, alkalinity, calcium hardness, cyanuric acid (CYA), and copper level.
CYA above 100 ppm: Partial drain and refill is the only effective way to reduce CYA. No chemical treatment removes CYA.
Copper above 0.3 ppm: Confirms copper-based staining, proceed with sequestrant treatment.
Step 2: Partial Drain and Refill to Reduce CYA
Target: CYA 30-50 ppm (below 80 ppm absolute maximum).
Calculation: To reduce CYA from 120 ppm to 40 ppm requires draining approximately 67% of pool volume and refilling with fresh water (no CYA).
Practical approach: Drain 50% first, refill, retest. If CYA is still above 80 ppm, drain another 30-40%, refill.
Simultaneous copper reduction: Partial drain also reduces dissolved copper concentration proportionally = draining 50% removes 50% of dissolved copper, simplifying subsequent chemical treatment.
Step 3: Add Metal Sequestrant
Add chelating metal sequestrant (citric acid or EDTA-based products like BioGuard Pool Magnet, Jack’s Magic Blue Stuff, or similar) to hold remaining copper in solution rather than allowing it to precipitate as additional cyanurate.
Add before any chemical adjustment or shocking: Sequestrant must be in water before chlorine addition prevents further copper precipitation.
Circulate 24 hours without adding other chemicals.
Step 4: Adjust Water Chemistry Gradually
After sequestrant added and circulating, adjust:
- pH 7.4-7.6 (upper range – less corrosive, reduces further copper dissolution from fittings)
- Alkalinity 80-120 ppm
- Do not add chlorine shock yet
Step 5: Physical Stain Removal
Existing copper cyanurate deposits may dissolve partially as CYA and copper levels drop and water warms.
Ascorbic acid treatment: Dissolve 500 grams ascorbic acid powder (vitamin C) in a bucket of pool water, pour around the pool perimeter while the pump is running. Ascorbic acid reduces copper deposits, clearing staining. Allows 30 minutes circulation, observe staining. Repeat as needed.
Brush: After ascorbic acid treatment, brush surfaces to dislodge loosened deposits. Vacuum to waste.
Timeline: Purple cyanurate staining can take 2-4 weeks to fully clear even with correct treatment. There’s no quick fix. Patience is required. Avoid aggressive chemical additions that risk further copper reactions.
Step 6: Rebalance and Restart Normal Chlorination
Once CYA at 30-50 ppm and copper below 0.2 ppm (test with metal test kit):
- Add fresh calcium hypochlorite shock (non-stabilised – does not add more CYA)
- Balance all chemistry
- Maintain using unstabilised chlorine (liquid chlorine or calcium hypochlorite) for remainder of season to prevent CYA re-accumulation
Prevention
Manage CYA
Test monthly: CYA should be tested regularly. Most basic test kits do not include CYA. Use a dedicated CYA test kit or take a water sample to a pool supply shop.
Target 30-50 ppm: Never allow above 80 ppm. If approaching 70-80 ppm, perform 20-30% partial drain to dilute before it reaches problematic levels.
Reduce stabilised chlorine reliance: Trichlor tablets convenient but add CYA with every tablet. Consider switching primary chlorination to unstabilised liquid chlorine or calcium hypochlorite shock, using trichlor only when the pool is used lightly.
Avoid dichlor shock: Every bag of dichlor adds CYA. Use cal-hypo shock instead for regular shocking.
Use Only Chelated Copper Algaecides
Check label: Look for “chelated copper” or “EDTA-chelated” on algaecide labels. Non-chelated copper sulfate products (often cheaper, sold in supermarkets) should be avoided.
Apply correctly: Follow dosage instructions precisely. Copper algaecide overdose increases copper levels significantly increasing staining risk.
Alternative at closing: Use non-copper closing algaecide (polyquat 60% formulation) instead of copper algaecide to eliminate copper introduction at closing.
Address Copper Sources
Maintain pH 7.2-7.6: Acidic water (below 7.2) corrodes copper fittings and heat exchanger tubes releasing copper. Correct pH is primary prevention for heater-sourced copper.
Test fill water: If using a private well, test source water for copper, iron, and manganese before filling the pool. If elevated, add sequestrant during filling before adding any chlorine.
Inspect heater: Annual inspection of heat exchanger for corrosion pitting. Replace before corrosion advances to significant copper release.
Monthly sequestrant: Preventive monthly sequestrant addition keeps trace metals in solution preventing precipitation. Particularly important in areas with copper supply pipes or known metal content in fill water.
Natural Pool: Condition Cannot Occur
Natural swimming pools cannot develop copper cyanurate. The condition requires both cyanuric acid and copper, and neither is present in a properly maintained natural pool.
No cyanuric acid: Natural pools use no chlorine and therefore no stabilised chlorine products (trichlor tablets, dichlor shock) that release CYA. CYA is not added to natural pools for any purpose. The CYA level in a natural pool is zero.
No copper treatments: Natural pools use no copper algaecides and no copper-silver ionisation systems. Chemical treatment of any kind is absent – biological filtration through planted regeneration zones manages water quality without metal-based treatments.
Copper from equipment: Even if small amounts of copper enter a natural pool from fittings or fill water, with zero CYA, the copper cyanurate reaction pathway does not exist.
No CYA-related chlorine lockout: Natural pools experience none of the chlorine management complexity caused by CYA accumulation – no locked chlorine, no treatment gaps enabling algae or bacteria, no need for partial drains to reset CYA, no chemical cascade problems requiring management.
Oásis Biosistema designs natural swimming pools where biological filtration through planted regeneration zones eliminates the need for stabilised chlorine products, copper algaecides, and ionisation systems, making copper cyanurate formation chemically impossible and removing the seasonal CYA management cycle from pool maintenance entirely across Portuguese landscape installations.
Conclusion
Purple colouration in swimming pools is copper cyanurate. It’s a mineral chemical compound not algae, forming when cyanuric acid exceeds 100 ppm and reacts with non-chelated dissolved copper from low-quality copper algaecides, ionisation systems, corroding heaters, or copper-rich fill water, precipitating as purple crystalline dust on pool surfaces while leaving water relatively clear.
Treatment requires partial drain (50-67% of pool volume) to reduce CYA to 30-50 ppm, metal sequestrant addition before any further chemical adjustments, ascorbic acid (vitamin C) treatment to dissolve existing deposits, and switching from stabilised chlorine (trichlor, dichlor) to unstabilised liquid chlorine or calcium hypochlorite for remainder of season.
Natural swimming pools designed by Oásis Biosistema eliminate copper cyanurate entirely containing zero cyanuric acid and zero copper treatment chemicals, biological filtration through planted regeneration zones makes the chemical reaction pathway impossible while removing CYA management, partial drain cycles, and metal treatment requirements from the maintenance routine of Portuguese chemical-free pool installations.
FAQ
What causes purple algae in a pool?
Purple staining in a pool is usually not true algae. It is typically caused by manganese or copper in the water reacting with chlorine or oxidizers. This metal staining can appear purple, dark violet, or black and is often linked to well water or metal corrosion.
What color is pool algae when it dies?
When algae dies, it usually turns from green or yellow into a dull gray, white, or cloudy residue. It then breaks apart and gets trapped in the filter. True algae rarely turns purple – purple discoloration is more often a metal or chemical reaction issue.
Do copper and chlorine turn purple?
Yes, copper can react with chlorine or high pH levels to create purple, blue, or dark staining in pool water and on surfaces. This is not algae but a chemical reaction caused by dissolved metals oxidizing in the water.
How do I get rid of purple in my pool?
To remove purple staining, test for metals, balance pH, and use a metal sequestrant to bind copper or manganese. Avoid shocking until metals are treated. Then clean filters and maintain proper water balance to prevent reoccurrence and discoloration.
