Why Does My Hot Tub Feel Slimy, Sticky, Gritty, or Oily?


Spa water that is anything less than crystal clear poses a danger to both you and your hot tub. The good news is that the feel and appearance of the water are good indicators as to what’s causing the problem.

The three causes of slimy hot tub water are biofilm, bacterial growth (mold, mildew, algae), or a chemical imbalance. Water that feels rough, gritty, or sandpaper-like is due to a build-up of scale. Oily or greasy water is a sign of a build-up of contaminants and impurities.

This guide is jam-packed with everything you need to identify the cause and fix your water, as well as advice on how to prevent the problem from reoccurring in the future.

Your water feels slimy, slippery, or sticky

1. Biofilm

One of the major causes of slimy water in your hot tub is a build-up of bacteria and microorganisms that bind together to form biofilm. This bacteria can grow undetected in your hot tub pipes and can be seen in the filter or on interior surfaces.

The byproduct of biofilm is a greenish-white sticky residue that appears as a white film on the walls of your hot tub, causing your spa water to feel slippery.

Additionally, if your hot tub water is cloudy or has a bad odor, it may also be a sign of biofilm. Biofilm makes your sanitizers less effective and causes the water to become more acidic, which can cause health hazards and damage to your hot tub.

Although biofilm occurs naturally, its survival and growth depend on your hot tub’s environment. It’s best to treat biofilm as soon as possible as it only becomes harder to remove the longer it’s left.

How to fix biofilm

Shock the water – shocking your hot tub kills most of the bacteria living in the water. However, if biofilm is present, you’ll need to use four times the usual amount you use to shock your spa water.

After shocking the water, cover the tub, switch on the jets, and leave the hot tub to run for about an hour. Doing this ensures the chemicals spread throughout the interior workings of the spa, killing the bacteria within it.

Flush the pipes – After completing the shocking process, the next thing you want to do is use a line flushOpens in a new tab. to purge the biofilm hiding in the plumbing.

Removing biofilm needs to be done while your hot tub is full of water so that the chemicals can circulate through the pipes for an hour or so. Don’t be alarmed by all the grime that’s now inside your spa water. This is exactly what you want.

Now that the line flush has had enough time to circulate, it’s time to drain the hot tub completely. Once you’re done draining the hot tub, scrub the shell thoroughly using this chemical surface cleaner.

Ensure you rinse and dry the spa shell thoroughly after cleaning. Any biofilm left behind can cause the issue to come back again after you’ve filled with water.

How to prevent biofilm

  • Clean filters regularly and replace them every 12 months.
  • Practice proper hot tub maintenance.
  • Shower before using the spa.
  • Ensure water pH level is balanced.
  • Apply sanitizers regularly.
  • Cover the tub to prevent debris from entering.
  • Apply an enzyme-based disinfectant regularly.

2. Mold, mildew, and algae

Mold, mildew, and algae are very different organisms. Algae are plants, while mildew and mold are fungi. According to spa manufacturer, Hot Spring, sticky, slimy, or slippery sides are typically a sign of algae growth.

The Centers for Disease Control and Prevention (CDC) states that the walls of a properly sanitized hot tub should never be slippery or sticky.

Algae in a hot tub can take various forms and appear in different colors, such as green, yellow, white, or black. This can cause your spa water to appear greenish or cloudy. 

Mold and mildew can appear as floating white slime in your hot tub water or discolored black or white patches on your tub shell. They first appear on your hot tub cover before spreading to the water. 

Although mildew can be easily wiped off, mold is a bit tougher. Mold is a sign of a more serious infestation. It forms from lotions, soaps, and other substances that have entered your hot tub from your body. It’s often found growing in your spa’s jets, filters, or plumbing.

Mold, mildew, and algae are most likely to form in a hot tub that has been neglected. However, if you treat your water properly, you wouldn’t need to worry about them forming, even if your spa is not used frequently.

How to fix mold, mildew, and algae

The growth of mold, mildew, or algae in your hot tub can occur in different grades. For minor problems, you can start with adjusting your pH levels to bring your hot tub water back to the proper balance. 

This should be followed by shocking the water. However, if you want to make sure the problem is completely eradicated, you should drain your spa after shocking. 

Once drained, use a gentle cleanser to clean the hot tub shell and jet nozzles. You also want to clean your filters before refilling your spa.

In cases of extreme contamination, you’ll need to shock the water again after refilling and follow this by repeating the draining and cleaning process once more. This removes all traces of the contaminants and prevents future contamination.

How to prevent mold, mildew, and algae

  • Ensure your hot tub is covered to prevent algae growth. 
  • Ensure the filters are cleaned and replaced regularly. 
  • Follow the instructions of your water care system for testing and maintenance.
  • Test your water frequently.

3. Chemical imbalance

The last issue you might face is water that hasn’t been properly treated. Adding an excessive amount of some chemicals into your water can make it feel slimy or unpleasant to soak in.

You should also check if you’re using borates. While they’re great at buffering pH and making your water feel softer, using too much can lead to the water feeling slimy.

How to fix a chemical imbalance

  • Test your hot tub water regularly.
  • Ensure you add the proper chemicals to keep your water clean and balanced.
  • If adding chemicals doesn’t work, replace all or some of the hot tub water.

Your water feels rough, gritty, or sandpaper-like

4. Scale build-up

Scale build-up causes your hot tub water to feel rough, gritty, or sandpaper-like. White flakes (scale) occur when water that is rich in minerals like calcium is heated. The amount of calcium in your water source is measured using a calcium hardness scale.

Another cause of hot tub scaling is when your water’s pH levels are off. If the pH level is not maintained within the safe range, the water becomes alkaline, leading to the deposit of a hard white scale.

This can also cause build-up inside critical components of the hot tub, which can, in turn, damage the filters and other surfaces.

How to fix scale build-up

The first thing to do is get a good-quality water drop test kit to help you determine the hardness, alkalinity, and pH levels. Test kits are far more accurate than test strips.

Adjust the alkalinity levels to between 100-150 ppm and then adjust the pH to between 7.4-7.6. While adjusting, use your test kit to check the water every 4-6 hours to ensure your pH levels remain within range.

If the scale is fresh, you should be able to use a soft rag or nylon brush to remove it without draining the hot tub. 

If the scale is more firmly deposited, you’ll need to drain all of the water and scrub the hot tub shell with this stain and scale control cleaner. Before draining, use this line flushOpens in a new tab. to any scale inside the pipes.

While scrubbing, pay special attention to the hot tub walls and the area around the jets. When you’re done, ensure you rinse the tub out thoroughly before refilling to prevent foaming.

If your water source is high in calcium, you should attach an inline pre-filter to your garden hose. This helps remove most of the impurities and makes balancing the water easier and less time-consuming.

How to prevent scale build-up

After you’ve refilled your hot tub and balanced the water, add a stain and scale control to prevent a build-up of calcium.

Moving forward, ensure you monitor your pH levels by testing the tub water at least once a week. If your pH level is too high or low, use a pH balancing kit to make the necessary adjustments.

Your water feels oily and greasy

5. Contaminants

Although some of the things that enter your hot tub are removed by your filter, there are many contaminants that are too small to be dealt with. If you leave them for too long, they’ll end up forming a layer of scum on the surface of the water, causing it to feel oily and greasy.

The most common cause of this hot tub scum is bodily residue. Dead skin cells, hairs, and oils from your body can contaminate your spa water every time you take a soak. Another source of contaminants is personal care products. When you get into warm hot tub water, these products wash off your skin and form an oily scum. 

Another thing that causes hot tub scum is when the metals present in the water react with chlorine and then oxidize. This can lead to the formation of greenish hot tub scum in the water. Poor water chemistry and a dirty or worn-out filter can also cause scum to build up in your spa. 

The color of the scum is an indication of the root cause:

  • Brown scum forms when the water’s pH is too high, and the minerals react with the chlorine.
  • Green scum forms when there is a high copper or magnesium level in the water. 
  • Blue-green scum is caused by a combination of bodily fluids, personal care products, metals, and a dirty or worn-out filter.

How to fix contamination

Here is a list of ways you can fix a contaminated spa:

  • Use a fine-mesh skimmer to skim off most of the scum floating on the water surface. If there’s a lot, rinse the skimmer between passes.
  • Use these inexpensive floating sponges to absorb all the body oils, lotions, and other substances that have built up in the spa. 
  • If you notice a ring of scum around the hot tub shell at the water line, use a surface cleaner and Magic sponge to remove it without draining the tub.
  • Clean and disinfect the filter. If this doesn’t get the job done, replace them with a new one.
  • If you’ve done everything above and the problem persists, you’ll have to drain the spa and clean it thoroughly.

How to prevent contamination

The type of scum will determine the prevention method. Since brown scum is caused by high pH levels, simply keep the water balanced to prevent it from forming. If the pH level gets higher than 7.6, use a pH decreaser to lower it to the optimum range of 7.4-7.6.

To prevent green scum, use an inline hose pre-filter when filling your tub. This will keep a portion of the metals out of the water. 

You can also use a metal sequestrant to clump the metals together before they oxidize and form into a green scum. Clumping the metals together enables the filter to remove them.

Since blue-green scum is formed by a combination of various substances, there are several steps that will help you prevent it. The following steps are also helpful for preventing other types of scum:

  • Test the hot tub water regularly.
  • Shower before using your spa.
  • Shock your spa water regularly
  • Clean your hot tub regularly
  • Clean the filter regularly
  • If it appears your scum is from a mineral build-up, use a scale prevention product.

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