Why Your Pool Water Smells: Decoding Different Pool Odours

You approach your pool expecting that fresh, clean water scent – but instead you're hit with an unpleasant smell. Maybe it's a strong chemical odour, or something musty, or even a swampy smell that makes you think twice about swimming. Pool odours are your water's way of communicating problems, and different smells indicate different issues. Understanding what your nose is telling you helps diagnose and fix water quality problems before they become serious. Let's decode the most common pool smells and what they really mean.

The "Strong Chlorine" Smell: Not What You Think

What You're Actually Smelling

Here's the surprising truth: that overpowering "chlorine" smell that burns your nose and makes your eyes water? It's not from too much chlorine – it's actually from too little effective chlorine. What you're smelling are chloramines (also called combined chlorine), which are compounds formed when chlorine binds with contaminants like sweat, urine, body oils, cosmetics, and sunscreen.

The Chemistry Behind It

Free chlorine is your sanitiser – it's odourless in normal pool concentrations. When free chlorine encounters organic contaminants, it oxidizes them, forming chloramines in the process. Chloramines are weak sanitisers but strong irritants with that characteristic harsh "pool smell." They're what cause red eyes, itchy skin, and respiratory irritation that people mistakenly blame on "too much chlorine."

Why It Happens

Heavy pool usage without adequate chlorine creates ideal conditions for chloramine buildup. Pool parties, kids' swimming lessons, or busy weekends introduce massive amounts of organic contaminants. If your chlorine level is marginal (1-2 ppm) or your pool is overstabilised (high cyanuric acid blocking chlorine effectiveness), you can't break down contaminants fast enough. Chloramines accumulate, and that smell intensifies.

The Solution: Shock Treatment

You need to "break the chlorine lock" by shocking – raising free chlorine to 10-20 ppm to oxidize and destroy chloramines. Use non-stabilised chlorine (liquid chlorine or cal-hypo) for shocking. Run your pump continuously for 24 hours and don't swim until chlorine drops below 5 ppm. The smell should disappear, replaced by fresh, odourless water. If it returns quickly, you have underlying issues: inadequate daily chlorination, high cyanuric acid, poor circulation, or excessive bather load.

The Musty or Mouldy Smell

What It Indicates

A musty, earthy, or mouldy odour suggests organic growth – algae, bacteria, or biofilm developing somewhere in your pool system. This smell is particularly noticeable in the morning before the pool has been used, or when first turning on the pump after it's been off overnight.

Common Causes

Inadequate sanitisation allows bacteria and algae spores to colonize surfaces. Biofilm – a slimy bacterial layer – often develops in low-flow areas: behind pool lights, in pipe dead-ends, inside filter housings, or on neglected pool equipment. In Australian pools, warm water temperatures (above 28°C) accelerate biological growth, making this smell more common during summer.

Old filter media, particularly sand that hasn't been changed in 7+ years, harbours bacteria and produces musty odours during backwashing. The filter itself becomes a breeding ground rather than a sanitising tool.

The Solution: Deep Clean and Sanitise

Shock the pool with chlorine to kill active algae and bacteria. Brush all surfaces thoroughly, paying special attention to steps, ladders, behind lights, and corners where biofilm hides. Clean or backwash your filter, or consider replacing old filter media. For persistent smells, you may need to remove and acid-wash the filter, or treat with a filter cleaner designed to remove biofilm and oils.

Check and clean your chlorinator cell, pump basket, and skimmer baskets – anywhere organic matter can accumulate and decay. Ensure you're maintaining adequate daily chlorine levels (1-3 ppm minimum, higher during hot weather) to prevent regrowth.

The Swampy or Stagnant Water Smell

What It Indicates

A swamp-like odour – similar to pond water or stagnant puddles – indicates serious water quality problems. This smell means your pool has minimal or zero effective sanitisation, allowing bacteria, algae, and organic decomposition to proceed unchecked.

Common Causes

Complete chlorine depletion is the usual culprit. Perhaps your chlorinator failed, you ran out of chlorine tablets, your salt cell died, or you simply neglected maintenance during a holiday. Heavy rain diluting chlorine levels combined with organic debris (leaves, soil, pollen) creates perfect conditions for this smell to develop.

High phosphate levels feed algae growth, contributing to organic load and swampy smells. Overstabilised pools (high cyanuric acid) where chlorine is present but completely ineffective also develop this characteristic odour as bacteria and algae flourish despite adequate chlorine readings.

The Solution: Aggressive Treatment

This requires serious intervention. First, test and balance pH and alkalinity – you can't sanitise effectively if these are wrong. Then shock heavily with non-stabilised chlorine, aiming for 20-30 ppm. You may need multiple doses over 2-3 days. Brush thoroughly twice daily to break up algae and biofilm. Run your filter 24 hours continuously, backwashing or cleaning frequently as it captures dead algae and debris.

Consider adding an algaecide (preferably non-phosphate based) after chlorine shock has begun working. Test phosphates and treat if elevated. In severe cases, you might need to partially drain and refill if cyanuric acid is prohibitively high or if total dissolved solids have accumulated beyond manageable levels.

The Rotten Egg or Sulphur Smell

What It Indicates

A rotten egg smell indicates hydrogen sulphide gas, typically from sulphur-reducing bacteria in your pool system. This distinctive odour is unmistakable and particularly unpleasant.

Common Causes in Australian Pools

This is particularly relevant for Western Australian pool owners using bore water. Bore water often contains sulphates and sometimes sulphur-reducing bacteria. When chlorine levels drop, these bacteria thrive in anaerobic (low-oxygen) conditions, producing hydrogen sulphide gas with that characteristic rotten egg smell.

Sulphur bacteria also colonize pool equipment, particularly heaters, heat pumps, and solar heating systems where warm, low-flow conditions exist. The smell often intensifies when you first turn on heating equipment that hasn't been used recently.

The Solution: Shock and Equipment Treatment

Shock chlorinate the pool to kill sulphur bacteria. You may need sustained high chlorine levels (10+ ppm) for several days. If the smell originates from heating equipment, you'll need to isolate and shock-treat that system specifically – sometimes requiring circulation of chlorinated water through the heater with valves closed to the main pool.

For persistent problems, consider professional cleaning of heat exchangers or solar panels where bacteria hide. Ensure consistent chlorine levels moving forward – sulphur bacteria only thrive when sanitisation lapses. If using bore water, test it before adding to your pool and consider pre-treating or diluting with scheme water.

The Chemical or Plastic Smell

What It Indicates

A sharp chemical smell (not chlorine-related) or plastic odour suggests equipment problems, chemical reactions, or contamination rather than water quality issues.

Common Causes

Overheating pool equipment – particularly pumps running dry or bearings failing – produces a burning plastic or electrical smell. Chemical spills or improper chemical mixing near the pool can create various chemical odours. Adding incompatible chemicals simultaneously (like chlorine and acid) can cause reactions producing chlorine gas with a sharp, dangerous smell.

New pool equipment, particularly cheaper plastic components or imported items, sometimes off-gas chemicals during initial use, creating temporary plastic odours that dissipate over weeks.

The Solution: Inspect Equipment

Investigate immediately – equipment smells can indicate dangerous conditions. Check your pump for proper operation, adequate water flow, and normal temperature. Inspect all equipment for overheating, burnt components, or chemical residues. Never mix pool chemicals directly together, and always add chemicals to water, never water to chemicals.

If new equipment is off-gassing, ensure adequate ventilation and contact the supplier if odours persist beyond a few weeks – some products are simply unsuitable for pool use.

Australian-Specific Smell Issues

Eucalyptus and Native Vegetation

Pools surrounded by Australian natives, particularly eucalypts, sometimes develop an earthy, slightly medicinal smell from oils and tannins leaching from falling leaves and bark. While not harmful, these organic compounds consume chlorine and contribute to cloudy water and that distinctive smell.

Bore Water Contributions

Western Australian bore water introduces not just sulphur issues but also iron, manganese, and other minerals that can create metallic or earthy smells, particularly when combined with chlorine. These smells often intensify after topping up the pool with bore water.

Heat and Evaporation

Australian summer temperatures accelerate chemical and biological processes. What might develop slowly in cooler climates can appear suddenly in 35-40°C weather. Evaporation also concentrates contaminants, intensifying smells as water levels drop.

Prevention Strategies

Preventing pool smells is easier than correcting them:

• Maintain consistent daily chlorine levels (1-3 ppm minimum)
• Shock weekly during peak usage or after heavy swimming
• Test water chemistry weekly, adjusting pH and alkalinity promptly
• Encourage pre-swim showers to remove oils, lotions, and contaminants
• Skim and remove organic debris daily before it decomposes
• Ensure adequate filtration time – 6-8 hours minimum daily in summer
• Clean or backwash filters regularly, replace filter media as needed
• Keep phosphate levels below 100 ppb to starve algae
• Maintain proper cyanuric acid levels (30-80 ppm depending on pool type)
• Don't let chlorine deplete completely – ever

Fun Fact: The "pool smell" that most people associate with swimming pools is entirely artificial and unnecessary – properly maintained pools should be virtually odourless! That characteristic harsh smell you remember from childhood public pools was actually chloramine pollution from inadequate sanitisation combined with high bather loads. A single swimmer introduces approximately 30-50ml of body fluids (sweat and small amounts of urine) plus oils, cosmetics, and contaminants into pool water during an hour of swimming. Multiply that by dozens of swimmers in a public pool, add inadequate chlorine, and you get massive chloramine buildup – hence that overpowering "pool smell." Your home pool, properly maintained, should smell like nothing at all – just clean, fresh water.

The Bottom Line

Pool smells are diagnostic tools – your water communicating problems that need attention. That strong "chlorine" smell paradoxically means you need more effective chlorine, not less. Musty smells indicate biological growth requiring sanitisation and cleaning. Swampy odours mean serious sanitisation failure needing aggressive treatment. Rotten egg smells point to sulphur bacteria, particularly relevant for WA bore water users. Chemical or plastic smells warrant immediate equipment inspection.

In Australia's challenging climate – intense UV, high temperatures, long swimming seasons, and diverse water sources – maintaining odourless pools requires consistent attention to chemistry and cleanliness. Test regularly, maintain proper chlorine levels, shock after heavy use, and address problems promptly when smells appear.

Remember: a properly maintained pool should smell like nothing at all. If your nose detects something wrong, trust it – investigate, test, and treat accordingly. Your pool water is talking to you through those smells. Listen, decode the message, and respond appropriately. Clear, odourless water isn't just more pleasant – it's confirmation that your pool chemistry is balanced, your sanitisation is effective, and your pool is truly safe and inviting for swimming.