Why Does My Hot Tub Smell Bad? (8 Common Odors)

Trying to identify the cause of that awful smell coming from your hot tub isn’t all that easy. Oftentimes, there are even several possible causes. This article addresses the most common unwanted odors, with guides on how to remedy the problem.


If your hot tub smells fishy, there are several things that could be causing the smell. One of which includes chloramines, which is the combination of chlorine and ammonia. 

The ammonia smell is a sign that the hot tub hasn’t been adequately sanitized. And without proper sanitization, bacteria will begin to grow in your hot tub, causing it to smell like rotten fish or a pond.

Another common cause of a fishy smell in your hot tub is algae growth. If you are relying on city water, fishy, musty, and earthy smells indicate an algae bloom has occurred, producing geosmin, the culprit behind the smell. Algae can also grow directly in your hot tub. 

If your water has low pH levels, this can also cause your hot tub to smell fishy. You can use a test strip to determine whether pH is the cause of the bad smell in your spa.

  • Note: a fishy smell may also be perceived as decaying/rotten fish; fish pond; ammonia; musty; or earthy.

How to fix chloramines

If the fishy smell in your hot tub is caused by chloramines, the best way to prevent it is by adding a chlorine-based shockOpens in a new tab. to the water to break down the chloramines.

You need to maintain an appropriate sanitizer level in your spa water. For chlorine sanitizers, maintain a level of 1-3 ppm. Meanwhile, for bromine sanitizers, maintain a level of 3-5 ppm.

How to fix an algae bloom

To fix an algae bloom and eliminate the musty smell in your hot tub, you’ll have to shock your hot tub with a chlorine-based shock.

Another way to fix an algae bloom is by cleaning the filters and circulating the water regularly. In addition, you should change the filters regularly. Dirty filters cause algae to build up, which can lead to a musty smell.

Rotten eggs

The smell of rotten eggs in your hot tub is most likely caused by hydrogen sulfide gas (H2S). Hydrogen sulfide gas is not harmful; however, the smell can be quite noxious. It is caused by decaying organic matter or by bacteria called biofilm.

Biofilm may be present in your hot tub water but is more likely in the plumbing and equipment. So, you want to ensure the dead animal smell is coming from your spa and not from your drains.

Organic matter can come from various sources. For instance, when we get into the hot tub, we introduce all sorts of organic matter, such as oils and dead skin cells. Additionally, when you leave your lid off for a while, bugs and plant life can enter your spa.

Another cause of the dead animal smell is the corrosion of metallic parts. When metals corrode, they release excess electrons. 

The sulfur (sulfate ions, SO4-) in the water combines with these electrons and converts to sulfides (S) and hydrogen sulfide gas. This process is known as a sulfate-reduction reaction.

  • Note: a rotten egg smell may also be perceived as sulfur; cabbage; stinky feet; dead animal; or sewage-like.

How to fix

To eliminate the rotten egg smell in the hot tub water, change the water every 3 months and use this Ahh-Some line flushOpens in a new tab. to clean out any biofilm lurking in the pipes.

You can also eradicate this sewage-like smell by rinsing your filters. Do this every week if you use the hot tub 3 or more times a week and every fortnight if you use it less.

Additionally, clean the filters with a chemical sprayOpens in a new tab. once and month and use a professional-grade chemical soakOpens in a new tab. to perform a deep filter clean every 3 months. Replace the filters annually.

Wet dog

If your hot tub water has a smell similar to that of a wet dog, then the cause is more likely from your water supply or water pipes.

Naturally, there are minerals and metals present in your water, but sometimes large concentrations of metal in your pipes can affect your water’s smell. Copper, lead, iron, sodium, and magnesium can build up over time in your plumbing and taint your water supply.

Bacteria and other organisms can also build up in your water supply. According to the Salt Institute, both copper and PVC pipes can collect mineral deposits over time and serve as a breeding ground for bacteria. This can cause your water to have an unpleasant aroma. 

Furthermore, the chemicals used to remove bacteria and other contaminants can also cause your water to have a wet-dog smell. Yet another cause could be environmental contaminants based on where you live.

  • Note: a wet dog smell may also be perceived as fruity, feces, or sulfur.

How to fix

If you receive water from a public treatment facility, you probably have nothing to worry about. This is because your water is tested regularly and follows strict health guidelines. 

However, if your hot tub water comes from a private well, that wet dog smell may be caused by a build-up of bacteria in the groundwater. To fix this issue, you should have your water supply tested by following the steps recommended by the United States Environmental Protection Agency.


When you smell chemicals or bleach in your hot tub, the first thing that comes to mind is chlorine. However, that chlorine-like smell is caused by chloramines and not chlorine.

Chloramines build up in a hot tub when it is improperly treated. They form when chlorine combines with ammonia (oils, sweat, and urine) that are introduced into the spa by the bathers. You can prevent chloramines from forming in your spa by showering before you use your hot tub. 

Another reason your hot tub smells like chemicals could be that the pH levels are not balanced correctly. You can prevent this by testing your spa water and balancing the water chemistry regularly.

Below are safe levels for your hot tub:

  • Free chlorine – 1-3ppm
  • Alkalinity – 80-150ppm
  • pH – 7.2-7.8ppm
  • Calcium hardness – 175-250ppm

In addition, public water suppliers usually chlorinate water to prevent bacterial growth. So if you’re connected to a public water supply, your water may smell of chlorine, but not so much that you’d notice it in a hot tub.

  • Note: a chemical smell may also be perceived as chlorine or bleach.

How to fix

If the bleach smell is caused by chloramines, you can fix this by super-chlorinating the hot tub water with a chlorine-based shockOpens in a new tab.. This will break the bond between chlorine and ammonia. Add enough to raise the chlorine levels to 10 ppm of chlorine and run your jets for a while to let it off-gas.

If the chlorine smell is caused by a pH imbalance, you’ll need to add a pH increaserOpens in a new tab. or pH decreaserOpens in a new tab. to adjust the pH levels accordingly.

Mold and mildew

Generally, hot tubs develop a mildew smell due to excessive mold and algae growth in the spa water. One of the main causes of mold growth is the lack of sanitizer.

Another cause of mold growth is worn-out filters. If your hot tub filters are worn-out, they won’t cleanse your water of contaminants. 

Your hot tub cover can also spread mold to your spa water. Therefore, if you notice any mold or slime on your hot tub cover, thoroughly clean and disinfect it. However, if you notice mildew on the cover, you may need to air or replace it.

Furthermore, mold in your hot tub’s plumbing can spread to the water, usually in the form of white flakes. When looking out for mold, most people expect it to have a particular smell. However, there are various species of mold, each having different odors, textures, and colors. 

Some molds may produce an earthy smell, some may produce sweeter smells, while some smell like rotten meat and fermenting alcohol. Additionally, a single species can produce wildly different smells. It is important to keep this in mind when trying to detect mold in your hot tub.

  • Note: a mold and mildew smell may also be perceived as musty; earthy; stale; grassy; sweet; or minty.

How to fix

You can fix the mildew and mold smell by sanitizing your hot tub water regularly. In addition, since a worn-out filter leads to mold growth, you must rinse it every week, clean it with a chemical sprayOpens in a new tab. once per month, and deep clean them in a professional-grade chemical soakOpens in a new tab. every 3 months. Furthermore, ensure you replace your filter annually.

Fixing the mold in your plumbing typically requires hyperchlorination with a chlorine-based shockOpens in a new tab., purging the pipes with this line flushOpens in a new tab., and draining and refilling your spa with fresh water.


When a hot tub smells of ammonia, it is due to oil, sweat, and urine from bathers reacting with the chlorine in the water. This reaction turns the chlorine into chloramines (combined chlorine) and reduces its sanitizing power in the spa water.

The ammonia smell indicates that the water hasn’t been properly sanitized or is due for another treatment. As a general rule, never use a hot tub that smells like ammonia. The ammonia smell indicates something is not right and may not be safe or pleasant to soak in.

  • Note: an ammonia smell may also be perceived as fishy; urine; or sweat.

How to fix

To eliminate the ammonia in your hot tub, you’ll have to eliminate the chloramines by super-chlorinating the spa water with chlorine shockOpens in a new tab.. As discussed earlier, you can do this by adding enough to raise the chlorine levels to 10 ppm.

After adding the chlorine, leave the spa cover open and run the jets, and then test and adjust the chemicals as needed. If the smell persists, you may have to drain the hot tub water and replace it, balancing the water chemistry again afterwards.


If you notice a sweet smell in your hot tub, there are two possible causes: antifreeze or ozone. Antifreeze, such as ethylene glycol or propylene glycol, tastes and smells sweet. So when if you’ve used it to winterize your hot tub, any remaining antifreeze in the plumbing could be tainting the water.

If you sense a sweet odor in the air, it is likely to be ozone, which is the result of a nearby storm. During a storm, lightning separates the oxygen atoms within gaseous oxygen. These atoms can form a new gas composed of three oxygen atoms. This gas is known as ozone gas.

According to Scientific American, the smell of ozone is both sweet and pungent. So if you smell such an odor, avoid your hot tub or any body of water as you run the risk of electrocution.

Apart from the potential to be struck by lightning, higher concentrations of ozone can cause serious damage such as inflammation in your respiratory tract. Additionally, ozone can make it more difficult to breathe. It can also worsen existing lung conditions like asthma and emphysema.

Since different molds have different odors, a sweet or yeasty smell in your hot tub could be a sign of mildew or mold.

  • Note: a sweet smell may also be perceived as fruity; fermenting fruit; yeast; or vinegar.

How to remove antifreeze

If the cause of the sweet smell is antifreeze, you’ll need to flush it out of your hot tub completely before you fill your spa with water. The easiest way to do this is to spray water directly into each jet to rinse out the antifreeze. 

You’ll also need to spray down the seats and footwell with water. Ensure you use a high-pressure stream of water for spraying. You can get this from your normal garden hose with a nozzle sprayerOpens in a new tab. attached.


The primary cause of a plastic smell in your hot tub is when biofilm is present inside the plumbing. This biofilm usually forms when the manufacturer is testing the system to make sure that there are no leaks.

During the testing process, a small amount of water gets passed through the plumbing system and stays there for a long period before the spa gets used by a customer or is treated using chlorine. It is this water that causes the growth of biofilm.

It’s one thing for your hot tub to smell like plastic and a whole other issue if it smells like burning plastic. This smell is typically associated with your heater or extremely high temperatures that can burn your pipes. 

Check for melted surfaces around your heater’s area. Also, check if your heater is faulty. In addition, ensure there are no loose electric wires as well. 

If none of these seems to be the cause of the plasticky smell, you might have overheated the hot tub, causing it to melt some pipes or rubbers attached to the spa.

How to fix

To fix this plastic smell, you’ll need to get rid of the biofilm. You can do this by thoroughly flushing the hot tub system with Ahh-Some plumbing and jet cleanerOpens in a new tab.. Switch on the jets and allow plenty of time for the flushing solution to take effect.

The dirt and debris from the system will end up floating around in the hot tub water. Therefore, you’ll need completely drain the spa, clean the hot tub shell with a professional-grade chemical surface cleanerOpens in a new tab., refill the tub, and balance the water and chemicals.

Joshua Milton

Joshua Milton is a seasoned hot tub enthusiast. With many years of experience in the industry, he offers valuable insights on hot tub maintenance, health benefits, and relaxation techniques.

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