White algae in pools is most commonly white water mould (Saprolegnia spp.). It’s a chlorine-resistant oomycete organism appearing as white or grey slimy film on pool surfaces, floating chunks resembling wet tissue paper, or white coating inside pipes and filter equipment. Despite the name, white water mould is not a true fungus nor algae but a water mould related to brown algae, forming protective biofilms composed of cellulose, glucans, and chitin that resist standard chlorine levels. Treatment requires triple or quadruple shock (30+ ppm chlorine), thorough brushing of all surfaces, complete filter cleaning and decontamination, and cleaning or replacement of all pool accessories (toys, floats, hoses) that harbour biofilm between treatment cycles.
This guide covers white water mould identification versus calcium deposits and other white substances, pink slime co-occurrence (Serratia marcescens bacteria), causes (low chlorine, organic matter, biguanide sanitiser systems), health risks, step-by-step treatment protocol, filter decontamination, equipment cleaning, prevention schedule, and natural pool alternative where beneficial competing microorganisms prevent pathogenic biofilm establishment.
Key Takeaways
- White water mould ≠ true algae or fungus – oomycete organism forming chlorine-resistant biofilm
- Appears as white slimy film or tissue-like chunks – on walls, pipes, filter, accessories
- Chlorine resistant – standard maintenance doses ineffective, requires 30+ ppm shock
- Almost always co-occurs with pink slime – Serratia marcescens bacteria inhabits same biofilm
- Filter must be decontaminated – mould in filter re-seeds pool after shock treatment
- All accessories must be cleaned – floats, toys, brushes harbour biofilm through treatment
- Common in biguanide (PHMB) pools – more frequent when chlorine is not primary sanitiser
- Recurrence likely without prevention – maintained free chlorine 3+ ppm and organic load reduction required
In a Natural Pool, White Algae Isn't on the List
Identification: White Water Mould vs Other White Substances
White Water Mould Characteristics
Appearance: White, grey, or off-white slimy coating on pool surfaces. Free-floating pieces described as resembling wet toilet paper, soggy tissue, or chunks of white slime. Translucent to opaque.
Texture: Slimy, slightly stringy. Dissolves when handled but reforms. Distinct from crystalline mineral deposits.
Location: Wall surfaces, behind ladders and steps (low-circulation areas), inside skimmer, on return jet fittings, coating hoses and plumbing. Filter media (cartridge pleats, sand media surface, DE grids) often heavily colonised.
Smell: Earthy, musty odour distinct from normal pool chemical smell.
Distinguishing from Other White Pool Problems
Calcium carbonate scale: Hard, crystalline, rough texture. Does not dissolve when rubbed. Adheres firmly to surfaces. Appears at the waterline as a crusty white deposit, not floating chunks. Tests with pH drop. Drops of muriatic acid cause fizzing on calcium scale but not on mold.
Calcium floc: White cloudiness throughout water (not film), caused by calcium oversaturation when flocculent added. Settles to the pool floor as white powder layer. Not slimy, no surface film.
Diatomaceous earth in water: White fine powder on pool floor from torn DE filter grid. Not slimy, no surface film, no musty odour.
Biguanide (PHMB) product residue: White milky cloudiness in water, not surface film. Common when biguanide shock products overdosed. Distinct from mold.
Sunscreen and lotion film: Oily white film on water surface, appears in rings at waterline. Dissolves with enzyme treatment, no musty odour, no attached surface colonies.
White Water Mould Biology
Classification: Saprolegnia spp. and related genera (Oomycota class). Classified historically as fungi but genetically distinct. It’s more closely related to brown algae and diatoms than true fungi.
Biofilm structure: Saprolegnia creates protective biofilm composed of cellulose, glucans, and chitin and has the same structural materials as black algae biofilm. This matrix shields the organism from chemical penetration.
Chlorine resistance: Standard maintenance chlorine (1-3 ppm) insufficient to penetrate biofilm. The organism survives inside a protective matrix while outer layers bleach white.
Reproduction: Produces microscopic spores dispersed through water. Spores present throughout pool water and plumbing even when visible mold is absent. Requires decontamination of the entire water system (not just visible surface colonies) to prevent regrowth.
Preferred conditions: Low or absent sanitiser, presence of decaying organic matter (dead leaves, hair, skin cells, insects), low-circulation areas, warm water. Does not require green or cloudy water and can be established in visually clear pools with inadequate sanitiser.
Pink Slime Co-Occurrence
White water mould and pink slime (Serratia marcescens bacteria, sometimes also Methylobacterium species) almost always occur together. White water mould biofilm provides habitat and nutrient source for pink slime bacteria where the two organisms establish a symbiotic relationship.
Pink slime appearance: Pink, salmon, or coral-coloured slimy growth in low-circulation areas behind ladders, under skimmer lids, at waterline corners, or inside return jet bodies.
Health risk from pink slime: Serratia marcescens is an opportunistic pathogen causing urinary tract infections, wound infections, and respiratory infections particularly in immunocompromised individuals. More dangerous than white water mould itself.
Treatment implication: Must treat both organisms simultaneously. White water mould treatment (chlorine shock) only partially addresses pink slime as pink bacteria requires specific biocide or algaecide labelled for pink bacteria/slime in addition to shock treatment.
Causes and Conditions
Primary Cause: Inadequate Sanitiser
Low free chlorine: Free chlorine below 1 ppm allows white water mold to establish. Organic biofilm builds up in low-circulation areas first, then spreads.
Chlorine demand exceeds supply: Heavy bather load, warm water, direct sunlight, high organic input (leaves, pollen, insects) consume chlorine faster than production or dosing replenishes it.
Biguanide (PHMB) Sanitiser Systems
Higher incidence: Pools using polyhexamethylene biguanide (PHMB, marketed as Baquacil, Soft Swim, Revacil) as primary sanitiser experience significantly higher white water mould incidence than chlorine pools.
Reason: PHMB is effective against bacteria and algae but does not prevent white water mould biofilm establishment in the same way chlorine does. PHMB systems rely on hydrogen peroxide as a supplemental oxidiser. This combination is less effective against Saprolegnia than chlorine.
Switching from biguanide: Pools converting from biguanide to chlorine frequently experience white water mould outbreak during transition. PHMB residue and existing biofilm are not fully addressed by initial chlorine addition.
Organic Matter in System
Decaying material as food source: White water mould is parasitic on decaying organic matter such as dead leaves in skimmer, organic debris in filter, hair and skin accumulation in plumbing. Removing organic food sources reduces establishment risk.
Filter condition: Heavily soiled filter media harbours white water mould colony protected from pool water chlorine. Filter re-seeds pool after each treatment cycle if not decontaminated.
Pool Accessories
Floats, toys, hoses: Porous or soft surfaces on pool accessories harbour white water mould and pink slime biofilm. Accessories removed from the pool for winter or storage retain contamination, re-introduce mold on next season use.
Health Risks
White water mould: Minimal direct health risk to swimmers. The organism dies immediately on leaving water and drying. Causes no established human infection in healthy individuals. May cause skin irritation in sensitive individuals.
Pink slime (Serratia marcescens): Greater health concern. Opportunistic pathogen causing:
- Urinary tract infections (UTI) – particularly via contaminated water contact
- Wound infections through cuts or abrasions
- Respiratory infections from inhaling aerosolised contaminated water
- Eye infections from direct water contact
High-risk groups: Immunocompromised individuals, people with open wounds, urinary catheters, or respiratory conditions should avoid swimming in pink slime-contaminated pools.
Practical guidance: Do not swim in visibly mould or slime-contaminated pools. Treat before next use.
Step-by-Step Treatment Protocol
Step 1: Balance Water Chemistry
Test and adjust pH 7.2-7.4 (lower end aids sanitiser effectiveness), alkalinity 80-120 ppm, calcium hardness 200-400 ppm. Correct chemistry before shocking as incorrect pH can dramatically reduce chlorine effectiveness.
Step 2: Brush All Surfaces Thoroughly
Using a stiff pool brush, scrub every surface like walls, floor, steps, behind ladders, and around fittings. Force brush into skimmer mouth, around return jet bodies, along waterline.
Brushing breaks the biofilm surface layer, exposing the inner organism to sanitiser. Do not vacuum yet because brushed material needs to enter a filter for decontamination first.
Step 3: Clean and Remove All Accessories
Remove every item from the pool including floats, toys, brushes, hoses, thermometers, and pool cleaning equipment. Scrub each item with a chlorine solution (50 ml household bleach per 10 litres water). Rinse thoroughly. Allow to dry completely in sunlight before reinstalling.
Vacuum hose interiors are particularly prone to harbouring mold. Soak the hose in chlorine solution, and flush it through.
Step 4: Shock Pool at Triple or Quadruple Normal Dose
Target: Free chlorine 30+ ppm for white water mould. Standard shock (10 ppm) insufficient to penetrate cellulose-glucan biofilm.
Dosage: Triple shock = 3× normal shock dose. Example: if the normal shock dose for your pool volume is 500 grams calcium hypochlorite, use 1,500 grams.
Shock type: Calcium hypochlorite (granular) or liquid chlorine. Dissolve granular shock in bucket before adding to avoid surface bleaching.
Add at night: UV destroys free chlorine rapidly. Evening addition allows chlorine to work overnight at full concentration.
Circulate: Run pump continuously 24 hours after shocking.
Step 5: Add Pink Slime Treatment (If Pink Slime Present)
Turn off pump before adding: Pink bacteria biocide or algaecide labelled specifically for pink slime/bacteria. Follow product dosage instructions. Brands include Pink Treat, Algeacide 60 Plus, and others.
Wait 24 hours before restarting the pump to allow product contact time with the biofilm.
Step 6: Decontaminate Filter Completely
This step is critical = mould colonising filter media will reseed pool repeatedly if not addressed.
Cartridge filter: Remove cartridges, soak overnight in filter cleaning solution (enzyme-based cleaner or 100 ml bleach per 10 litres water). Scrub pleats with a soft brush. Rinse thoroughly. If heavily colonised (mould visible between pleats), replace cartridge because cleaning does not fully decontaminate severely infected media.
Sand filter: Backwash vigorously, then add sand filter cleaner through skimmer (products containing enzymes or chlorine compounds that contact sand media during normal filtration cycle). Run 24 hours then backwash again. Consider sand replacement if mould is established for an extended period.
DE filter: Full teardown by removing the grids, scrub each individually with a cleaning solution, soak for 30 minutes, and rinse. Inspect for tears. Reload with fresh DE only after complete cleaning.
Step 7: Vacuum and Brush Again
After 24-48 hours of circulation, brush all surfaces again. Dislodges kill biofilm residue for filter collection. The vacuum pool removes settled debris.
Clean filter again after vacuuming as dead organic matter loads filter rapidly.
Step 8: Maintain Elevated Chlorine for One Week
Maintain free chlorine 3-5 ppm for a minimum 7 days after treatment. Higher-than-normal residual prevents re-establishment from any surviving spores in plumbing or filter.
Chlorine testing: Test daily during recovery week. Add chlorine as needed to maintain 3+ ppm.
Prevention
Maintain Free Chlorine 2-3 ppm
The most important prevention measure. Free chlorine above 2 ppm prevents biofilm establishment in all but extremely contaminated conditions.
Test twice weekly: Regular testing catches chlorine drops before mould can establish.
Stabiliser level: Cyanuric acid 30-50 ppm protects chlorine from UV degradation in outdoor pools. Above 100 ppm chlorine becomes ineffective, do a partial drain and refill to reduce CYA.
Reduce Organic Load
Skim daily, vacuum weekly, clean skimmer basket after each use, clean filter regularly. Less organic material available as a food source reduces mould establishment risk.
Improve Circulation
Direct return jets to maximise water movement in low-circulation areas (behind steps, corners). Run pump minimum 8-10 hours daily. Stagnant areas where water barely moves are primary establishment zones.
Clean Accessories Seasonally
At the start and end of each pool season, clean all accessories (floats, brushes, hoses, thermometers) with diluted chlorine solution. Store dry – moist stored accessories develop mold even during off-season.
Monitor After Opening From Winter
First 2 weeks after opening the pool from winter closure, test daily and maintain chlorine 3 ppm. Warm water, fresh organic material, and reactivating microorganisms create an ideal mold establishment window.
Natural Pool Alternative
Natural swimming pools prevent white water mold through competitive exclusion. The same principle is used in healthy natural ecosystems to prevent pathogenic organism dominance.
Beneficial bacterial colonisation: Regeneration zones in natural pools develop dense, diverse beneficial bacterial communities in gravel substrate and plant roots. These communities occupy the ecological niches that white water mould and pink slime bacteria exploit in low-sanitiser conventional pools.
Competitive exclusion: Established beneficial bacterial biofilms produce antimicrobial compounds and outcompete pathogenic organisms for nutrients and attachment surfaces. Saprolegnia requires decaying organic matter as a food source. Beneficial bacteria consume this organic matter first, reducing available substrate.
Plant root exudates: Aquatic plants (Typha, Phragmites, Juncus, Iris) release antimicrobial compounds from roots into surrounding water and substrate. Root zone chemistry hostile to pathogenic organisms while supporting beneficial bacteria.
No biguanide: Natural pools never use biguanide sanitisers. The primary conventional pool condition is associated with elevated white water mould but the risk is absent.
Continuous circulation through biological filter: Water continuously circulates through planted regeneration zones where biological activity maintains a balanced microbial community. Stagnant areas with low microbial competition — primary white water mould establishment zones — do not develop in properly circulated natural pool systems.
Oásis Biosistema designs natural swimming pools using planted regeneration zones where beneficial bacterial communities and aquatic plant root chemistry prevent pathogenic biofilm establishment through competitive exclusion, maintaining biologically balanced water without the sanitiser gaps that allow white water mould and pink slime to develop in conventional pool systems.
Conclusion
White algae in pools is predominantly white water mould (Saprolegnia spp.). It’s a chlorine-resistant oomycete forming protective cellulose-glucan biofilm appearing as white slimy surface film or tissue-like floating chunks, almost always co-occurring with pink slime bacteria (Serratia marcescens) which poses greater health risk through UTI and wound infection in susceptible individuals.
Treatment requires triple or quadruple chlorine shock (30+ ppm), thorough brushing of all surfaces, complete filter decontamination (cartridge soak, sand filter cleaner, DE grid scrubbing), cleaning all pool accessories in diluted bleach solution, and maintaining elevated free chlorine 3-5 ppm for one week post-treatment since standard maintenance doses cannot penetrate the organism’s protective biofilm matrix.
Natural swimming pools designed by Oásis Biosistema prevent white water mold establishment through competitive exclusion in planted regeneration zones where diverse beneficial bacterial communities and plant root antimicrobial compounds occupy ecological niches exploited by pathogenic organisms in sanitiser-deficient conventional pool systems.
FAQ
How do I get rid of white algae in my pool?
What many people call “white algae” is often white water mold. To remove it, thoroughly brush pool surfaces, clean the filter, shock the pool with chlorine, and maintain proper sanitizer levels. Persistent cases may require multiple treatments and filter cleanings.
Can you swim in a pool with white algae?
It’s best not to swim in a pool showing signs of white water mold or white algae. While it is not usually highly dangerous, it indicates poor water sanitation and may harbor other bacteria or contaminants.
Is it safe to swim in a pool with white water mold?
Swimming is not recommended until white water mold has been eliminated and water chemistry is balanced. The presence of mold suggests inadequate sanitizer levels, which can increase the risk of exposure to unwanted microorganisms.
Does white mold eat chlorine?
White water mold does not literally eat chlorine, but it often develops when chlorine levels are too low. Organic contamination and biofilms can consume chlorine rapidly, making it harder to maintain proper sanitizer levels and allowing mold to grow.
