A dehumidifier that smells like fish is more than just unpleasant — it can signal bacterial contamination, mould growth, or even an electrical fault inside the unit. That distinctive fishy odour is one of the most common (and most alarming) complaints from dehumidifier owners, and it has very specific causes that require targeted solutions.
Whether your dehumidifier smells like dead fish, stale seafood, or a faint marine odour, this guide explains exactly what’s causing it, whether it’s dangerous, and how to fix it step by step. We service dehumidifiers across Auckland and see this issue regularly — especially during our humid autumn and winter months.
What Causes a Fishy Smell in a Dehumidifier?
A dehumidifier fishy smell almost always traces back to one of three root causes: biological contamination, electrical component overheating, or stagnant water chemistry. Understanding which one you’re dealing with is critical, because the fix — and the urgency — differs for each.
1. Bacterial Biofilm and Organic Decomposition
The most common cause of a fishy smell from a dehumidifier is bacterial biofilm growing inside the water collection tank, on the evaporator coils, or within the drainage system. When bacteria colonies break down organic matter (dust, skin cells, pollen, and other airborne particles trapped by the unit), they produce organic amines — nitrogen-containing compounds that smell distinctly fishy.
The key chemical culprit is trimethylamine (TMA), the same compound responsible for the smell of rotting fish. Trimethylamine is produced when bacteria decompose choline and other organic nitrogen sources found in household dust. In a warm, moist dehumidifier environment, these bacteria thrive, and TMA levels can build up surprisingly fast.
Other amines produced during this decomposition include:
- Cadaverine and putrescine — diamines associated with decomposing organic matter
- Dimethylamine — a simpler amine with a pungent, fishy character
- Indole and skatole — produced by certain bacterial species in low-oxygen conditions
Biofilm is particularly problematic because it’s a structured colony of bacteria encased in a protective slime layer. This slime makes the bacteria resistant to simple rinsing — you need to physically scrub and chemically treat the affected surfaces to remove it completely.
2. Mould Colonies
Certain species of mould produce volatile organic compounds (VOCs) that have a distinctly fishy or marine character. Aspergillus and Penicillium species — both extremely common in Auckland homes — can produce metabolites that smell like stale fish, especially when growing on damp surfaces with access to organic nutrients.
Mould growth inside a dehumidifier typically occurs on:
- The evaporator coils (cold, consistently damp surfaces)
- The inside walls of the water collection tank
- The air filter, especially if it hasn’t been cleaned recently
- Drain hoses and fittings where standing water collects
Unlike bacterial TMA, mould-derived fishy smells tend to have a mustier undertone. If your dehumidifier smells like fish mixed with a damp, earthy note, mould is the more likely culprit.
3. Overheating Electrical Components
This is the cause most people don’t expect — and it’s the most serious. Overheating electrical insulation can produce a fishy or chemical odour that’s easily mistaken for biological contamination. Materials like Bakelite, certain plastics, and wire insulation coatings release organic compounds when they overheat, and many of these compounds are amines that smell distinctly fishy.
Common electrical sources of a fishy smell include:
- Failing capacitors — electrolytic capacitors can leak and produce a fishy chemical odour as they degrade
- Overheated wiring insulation — worn or pinched wires generate heat at the contact point
- Corroded circuit board components — humidity can accelerate corrosion of solder joints and traces
- Compressor relay overheating — the relay that starts the compressor can overheat if it’s failing
The key distinction: an electrical fishy smell tends to be sharper and more chemical compared to the organic, rotting character of bacterial TMA. It may also come and go with the compressor cycle. If you notice the fishy smell is strongest when the compressor kicks in and fades when it cycles off, electrical overheating is very likely the cause.
Important: If you suspect an electrical source, read our guide on burning smells from dehumidifiers as well — there’s overlap between fishy and burning electrical odours, and both require prompt attention.
4. Stagnant Water Chemistry
Even without heavy bacterial contamination, water left sitting in a dehumidifier tank undergoes chemical changes. Dissolved organic compounds from the air concentrate as water evaporates from the surface, and anaerobic (low-oxygen) conditions at the bottom of the tank can promote the formation of hydrogen sulphide and various amines.
This is particularly common when a dehumidifier is used intermittently — running for a few hours, then sitting idle for days with water still in the tank. The stagnant water becomes a concentrated soup of organic compounds that smells distinctly fishy or swampy when the unit runs again and air circulates over the tank.
Is a Fishy Smell from a Dehumidifier Dangerous?
The answer depends entirely on the cause:
Biological Causes (Bacteria/Mould) — Moderate Health Risk
Bacterial biofilm and mould in a dehumidifier mean you’re breathing in bacterial fragments, mould spores, and volatile organic compounds every time the unit runs. For most healthy adults, this causes irritation — sore throat, headaches, and a general feeling of stuffiness. For people with asthma, allergies, or compromised immune systems, the risk is more serious:
- Mould spores can trigger asthma attacks and allergic reactions
- Bacterial endotoxins (from biofilm) can cause respiratory inflammation
- Prolonged exposure to airborne amines can irritate the mucous membranes
Electrical Causes — Serious Safety Risk
A fishy smell from electrical components is a fire hazard. Overheating wiring, failing capacitors, and corroded connections can all lead to electrical fires. This is not something to monitor and hope it resolves — if you suspect an electrical cause, unplug the dehumidifier immediately and do not use it until it has been inspected by a qualified technician.
Signs the fishy smell is electrical rather than biological:
- The smell is sharper, more acrid, or has a plastic/chemical undertone
- It intensifies when the compressor runs and fades when it stops
- You notice the unit is unusually warm to the touch
- The circuit breaker has tripped while the unit was running
- You hear unusual buzzing or clicking from the unit
Stagnant Water — Low Risk
A fishy smell from stagnant water is unpleasant but generally low-risk for healthy adults. However, it indicates the unit needs cleaning, and leaving it will allow conditions to worsen toward full bacterial contamination.
Step-by-Step Fix: Eliminating a Fishy Smell from Your Dehumidifier
Follow this systematic process to identify and resolve the fishy odour. Work through each step in order — don’t skip ahead, as the cause determines the appropriate fix.
Step 1: Rule Out Electrical Causes First
- Unplug the dehumidifier and let it cool for 30 minutes
- Smell the unit while it’s off — if the fishy smell has faded significantly, it may be electrical
- Plug it back in and run it, paying attention to whether the smell correlates with the compressor cycle
- Feel the exterior of the unit — excessive heat in any one area (especially near the back or base) suggests an electrical issue
- If you suspect electrical causes, stop here — unplug the unit and arrange professional service
Step 2: Empty and Clean the Water Tank
- Remove the water collection tank and empty it completely
- Fill the tank with warm water and add 1 tablespoon of white vinegar per litre
- Let it soak for 30 minutes
- Use a bottle brush or soft-bristled brush to scrub every interior surface — pay special attention to corners, the float mechanism, and any textured areas where biofilm collects
- Rinse thoroughly with clean water
- For persistent odour, repeat with a solution of 1 teaspoon of baking soda per litre of warm water
Step 3: Clean or Replace the Air Filter
- Remove the air filter (usually behind a front or side panel)
- If it’s a washable filter, rinse under warm running water and gently scrub with a soft brush
- Allow the filter to dry completely before reinstalling — a damp filter will simply restart the problem
- If the filter is visibly discoloured, damaged, or the smell persists after cleaning, replace it
Step 4: Clean the Evaporator Coils
- With the unit unplugged, access the evaporator coils (the cold metal fins inside the unit)
- Use a soft brush or compressed air to remove loose debris
- Spray the coils with a mixture of equal parts white vinegar and water
- Allow the solution to sit for 15 minutes, then wipe gently with a soft cloth
- For heavy biofilm or mould, use a dedicated coil cleaner available from hardware stores
Step 5: Flush the Drainage System
- If your dehumidifier uses a continuous drain hose, disconnect it
- Flush the hose with a vinegar-water solution (50/50 mix)
- Use a pipe cleaner or flexible brush to scrub inside the hose
- Check the drain port on the unit itself and clean out any buildup
- Reconnect and ensure the hose has a continuous downward slope with no kinks or low points where water can pool
Step 6: Disinfect the Interior
- With all components removed and the unit unplugged, wipe down all accessible interior surfaces with a cloth dampened with white vinegar
- Pay attention to the area behind the tank and around the fan blades
- Allow everything to air-dry completely — at least 2-3 hours — before reassembling
Step 7: Test Run
- Reassemble the unit with all clean, dry components
- Run the dehumidifier in a well-ventilated area for 1-2 hours
- Check for any returning fishy odour
- If the smell persists after thorough cleaning, the cause is likely deeper — either mould has penetrated porous components, or there is an electrical issue that needs professional diagnosis
When to Call a Professional
While most fishy smells can be resolved with thorough cleaning, certain situations require professional service. Seek expert help if:
- The smell returns within days of thorough cleaning — this suggests the contamination is in areas you can’t access, such as inside the sealed refrigerant loop housing or deep within the fan assembly
- You suspect an electrical cause — any fishy smell that correlates with the compressor cycle, comes with unusual heat, or has a sharp chemical character needs electrical inspection
- The unit is tripping the circuit breaker — this is a clear electrical fault that requires immediate professional attention
- You see visible mould on components you can’t remove — if mould is visible on the evaporator coils or fan housing and you can’t access them for cleaning, professional disassembly is needed
- The dehumidifier is under warranty — attempting internal repairs yourself may void the warranty; check your documentation first
- The unit is more than 5-7 years old — older units with persistent electrical smells may not be worth repairing; consider hiring a newer, well-maintained unit instead
How to Prevent a Fishy Smell from Developing
Prevention is far easier than treatment. Follow these maintenance habits to keep your dehumidifier smelling clean:
Weekly Maintenance
- Empty the water tank every time it fills, or at least every 2-3 days even if it’s not full
- Rinse the tank with clean water when you empty it — don’t just pour out the water and replace the tank
- Check the air filter for visible dust buildup and brush it off
Monthly Maintenance
- Deep clean the water tank with vinegar solution as described in the step-by-step guide above
- Wash the air filter thoroughly and allow to dry completely
- Inspect the drain hose (if used) for kinks, blockages, or standing water
- Wipe down exterior vents and air intake areas
Seasonal Maintenance
- Before storing for summer: Run the unit on fan-only mode for 2-3 hours to dry out all internal components thoroughly before putting it away
- Before the first use in autumn: Clean the tank, filter, and accessible coils before the first run of the season
- Annual professional service: Consider having the unit professionally cleaned and inspected once a year, especially if it runs heavily through winter
General Best Practices
- Never leave stagnant water in the tank when the unit isn’t running
- Position the dehumidifier with adequate clearance on all sides for airflow
- Keep the room reasonably clean to reduce the organic particles entering the unit
- Use continuous drainage where possible to avoid water sitting in the tank
Auckland-Specific Tips
Auckland’s subtropical climate creates ideal conditions for dehumidifier-related odour problems. Here’s what local conditions mean for your unit:
Humidity and Mould Season
Auckland’s relative humidity regularly exceeds 80% from April through September, and frequently sits above 70% even in summer. This means dehumidifiers run harder and longer here than in most other parts of New Zealand, which accelerates biofilm development and organic buildup.
Peak mould season in Auckland runs from late March through October. During this period, your dehumidifier is working at its hardest and most likely to develop odour problems. Increase your cleaning frequency during these months — weekly tank deep cleans rather than monthly, and fortnightly filter washes.
Coastal and High-Humidity Areas
If you live in coastal Auckland suburbs — North Shore, Eastern Bays, West Coast areas — salt-laden air adds another dimension. Salt accelerates corrosion of electrical components, which can contribute to that fishy electrical smell developing sooner than expected. It also changes the chemistry of collected water, potentially making bacterial growth more vigorous.
For coastal homes, we recommend:
- More frequent coil cleaning (monthly rather than seasonal)
- Watching for corrosion on visible metal components
- Replacing units sooner if you notice performance degradation
Rental Properties and Inherited Dehumidifiers
Auckland’s rental market often means tenants inherit a dehumidifier that’s been sitting in a cupboard for months. If you’ve just moved into a rental and found a dehumidifier that smells like fish when you turn it on, give it a thorough clean using our step-by-step guide before writing it off. In many cases, a deep clean will resolve the issue completely.
If cleaning doesn’t help, or the unit seems old and in poor condition, hiring a commercial-grade dehumidifier is often more effective and economical than buying a new one — especially for short-term tenancies. Check our dehumidifier hire FAQ for details on how the process works.
How Fishy Smells Compare to Other Dehumidifier Odours
Different dehumidifier smells point to different problems. If the odour you’re experiencing doesn’t quite match a fishy smell, it may be one of these related issues:
- Rotten eggs smell — typically hydrogen sulphide from anaerobic bacterial activity in stagnant water; closely related to fishy smells but with a distinct sulphurous character
- Gas-like smell — may indicate a refrigerant leak, which is a different issue entirely and requires immediate professional attention
- Burning smell — overlaps with the electrical causes discussed above; if the fishy smell becomes more acrid or smoky, it’s likely progressing toward an electrical fault
- Urine-like smell — ammonia and amines from heavy bacterial contamination; chemically similar to fishy smells but with a sharper, more pungent character
Frequently Asked Questions
Why does my dehumidifier smell like dead fish?
A dead fish smell from your dehumidifier is typically caused by trimethylamine — a chemical compound produced when bacteria break down organic matter inside the unit. The warm, moist environment inside a dehumidifier is ideal for bacterial biofilm growth, and as these bacteria decompose dust, skin cells, and other particles collected from the air, they release fishy-smelling amines. Less commonly, the smell can come from overheating electrical components, which release amine compounds from degrading insulation materials.
Can a fishy smell from a dehumidifier be dangerous?
It depends on the cause. If the smell is from bacteria or mould, it poses a moderate health risk — particularly for people with asthma, allergies, or weakened immune systems. You’re essentially breathing in bacterial byproducts and mould spores. If the smell is from overheating electrical components, it’s a serious fire hazard. Unplug the unit immediately and have it professionally inspected before using it again.
How do I stop my dehumidifier from smelling like fish?
Start with a thorough deep clean: empty and scrub the water tank with white vinegar, wash or replace the air filter, clean the evaporator coils, and flush the drainage system. Allow all components to dry completely before reassembling. To prevent the smell returning, empty the tank regularly (don’t let water sit for more than 2-3 days), clean the tank weekly during heavy-use months, and wash the filter fortnightly.
Is a fishy smell from a dehumidifier an electrical problem?
It can be. Overheating electrical insulation, failing capacitors, and corroded wiring can all produce a fishy odour. The key way to distinguish electrical from biological causes is timing: if the fishy smell intensifies when the compressor kicks in and fades when it cycles off, or if the smell has a sharp, chemical quality rather than an organic rotting character, an electrical fault is likely. Check whether the unit feels unusually hot in any one area, and listen for buzzing or clicking sounds.
How often should I clean my dehumidifier to prevent fishy smells?
In Auckland’s humid climate, we recommend emptying and rinsing the water tank every 2-3 days, deep cleaning the tank with vinegar weekly during the April–October heavy-use season, washing the air filter every two weeks, and doing a full clean of all accessible components (including coils and drain system) monthly. Before storing the unit for summer, run it on fan-only mode for several hours to dry out the internals completely.