How To Get Rid Of Hot Tub Scum, Gunk Or Slime (Biofilm)



No one likes uninvited guests, but if you don’t pay attention to the water chemistry in your hot tub, you may just end up with some.

Hot tub scum is one such party crasher that’s sometimes referred to as gunk or slime (biofilm). Whatever you call it, it’s not only visually unappealing, but it upsets the tub water to a serious point of contamination.

Read on to learn more about this stubborn hot tub pest, and how you can keep it out of your soaker for good.

What Is Hot Tub Scum?

When hot tub scum is present, the water will have an oily look to it, and deposits will be caked onto the shell at the waterline.

Additionally, running the jets can cause the oil to froth and you’ll end up with a spa full of unpleasant looking foam.

Tubs can develop scum at times even if they’re well-kept, turning the water different shades of color in the process.

Brown, green, and aqua water are all possibilities, and the color can even help you figure out what caused it (more on that below).

Why You Have Hot Tub Scum

Hot tub scum isn’t caused by one specific issue, so the root of the problem is a bit harder to diagnose than other hot tub issues. Here’s a breakdown of the most common reasons you might be battling scum.

Your Sanitizer Isn’t Working Effectively

When the chlorine or bromine in the tub isn’t doing it’s job, “hot tub scum” will soon make an appearance.

A lack of sanitizer in the spa, either from being used up or not having enough of it in the first place, allows pollutants in the water to run rampant and eventually accumulate.

If that wasn’t enough, the more polluted your hot tub water is, the more volatile your pH level will be.

When your pH level is out of whack, it can make your water vulnerable to things like slime, hot tub scum and algae blooms. High pH levels can also cause minerals (such as iron) to react with chlorine in the tub, giving the water a brown tint.

Your Water Contains Too Much Copper

If the scum is turning the water green, there’s a good chance it’s due to a high copper level in the tub.

When copper mixes with water and chemicals, oxidation occurs, which turns the copper green.

This process is famously present on the Statue of Liberty, as well as on the roofs of historic buildings. These are all covered in an outer layer of copper which oxidized green when exposed to air.

Your Water Is Heavily Contaminated

Hot tubs sanitizers face a big challenge in keeping water clean, because every time someone enters a tub, it’s instantly polluted.

We’re talking about things that come from your body, such as dead skin cells, body oils, and even hair.

Many people also use personal care products that further contaminate the tub water. Sunblock, body lotion, hairspray, makeup, bathing suit detergent residue – the list goes on.

With such a small body of water, it doesn’t take much to turn it into a heavily contaminated spa.

You Have A Filtration Issue

If your swimming pool filter is compromised, you can develop a scum issue that manifests as aqua colored water.

The majority of tubs use a cartridge filter (or filters), which can last between 12 and 24 months. However, you have to rinse them every 2 to 3 weeks, and deep clean them every 3 months. 

With daily contaminants passing through the filter, they can become ineffective in a short period of time if you aren’t regularly cleaning them.

Also, as the months pass, these filters can tear or break down. By staying aware of its condition, you’ll be able to quickly swap it out for a new one before any major water problems occur.

How To Remove Hot Tub Scum (Step-By-Step)

The good news? Hot tub scum is treatable in most cases. However, the water may not be salvageable in extreme cases.

Here’s how to remove this crud from your spa:

1. Test And Balance Your Water

Any time you have an issue with scum, the water chemistry will need some attention. Start by getting it back to a balanced, neutral state.

Testing can be carried out using test strips, a liquid testing kit, or a digital testing meter. The levels you should be testing for and balancing are pH, total alkalinity (TA), calcium hardness (CH), and your sanitizer – either chlorine or bromine.

The ideal level of pH should be between 7.4 and 7.6, TA between 80 and 120 ppm, CH between 175 and 250 ppm, and chlorine between 1 and 3 ppm (bromine between 3 and 5 ppm).

2. Remove Gunk From The Surfaces

The scum will present itself on the surface of the water, so you should remove as much of it as you possibly can.

This can be done using a skimmer net to scoop up as much of it as possible. If there’s a lot, it’s a good idea to rinse off the net after each pass. 

Oil absorbing sponges are a good way to get some of the scum out of the water as well. Simply by placing them in the tub, the offending material can be soaked up into the sponge for easy removal.

3. Use A Biofilm Cleaner

Biofilm is also known as slime or white water mold. It’s a microorganism that sticks to surfaces that are regularly in contact with water.

In hot tubs, the microorganism covers itself in a pink slime to protect it from being destroyed by chlorine or bromine. Biofilm is common in spas because it thrives in warm, dark environments, and the pollutants in a typical tub help it thrive.

Bacteria like mycobacterium fortuitum, E. coli, as well as Legionnaires disease can all be the result of biofilm in the tub.

Using a cleaner specific to breaking down biofilm is necessary because sanitizers are ineffective against it.

Simply add the chemical to the water and run the pump for an hour. This gives the chemical time to destroy the biofilm in the tub, as well as in the pipes and filtration system.

4. Clean Or Replace Your Filter

Cleaning out your cartridge filter should be the next step. After scrubbing the tub and running the pumps, all the pollutants will have had a chance to get filtered out. This leaves the filter heavily compromised. 

Start by removing the filter from it’s housing. Depending on how dirty the tub was, you may need to fully replace the filter. 

If it looks salvageable, start by hosing it down to remove any superficial debris that’s caught in the pleats of the filter media.

If it still has a lot of debris on it, immerse the filter in a deep-cleaning chemical soak for 24 hours. Rinse it down before adding it back into your hot tub system.

5. Consider Doing A Full Drain

If your tub has a huge problem with contaminants in the water, and the sanitizer isn’t working effectively no matter how much you’re adding, it might be a good idea to do a full drain of the tub.

With scum and biofilm, sometimes this is the only way to restore your tub to its former glory.

The draining process isn’t too difficult, and you won’t have to second guess how clean the water is after you’ve swapped it all out.

How To Prevent Hot Tub Scum

Here’s a few tips for preventing the formation of hot tub scum in your spa.

Keep Your Water Balanced

When offending material enters a hot tub, the delicate balance of the water becomes upset.

Not only will this render your sanitizer useless, but the pH level, total alkalinity, and calcium hardness will be bumped from their neutral levels and thrown into disarray.

Keeping the water balanced at all times and shocking weekly will go a long way in preventing scum and slime from ever having the chance to form.

Maintain Your Filtration System

The role of sanitizers is to kill water contaminants, but it’s the role of the tub’s filter to remove these dead contaminants from the water.

The majority of tubs use cartridge filters (sand filters and DE filters are less common for spas), but you’ll need to maintain them on the regular so they don’t become bombarded and less effective over time.

Encourage Showering Before Use

Public pools require you to shower before use because your body is coated in pollutants that can be easily transferred into the water.

Things like body oils, skin cells, sunblock, lotions, soap and shampoo residue can all add to water problems and lead to scum formation. Showering beforehand will remove most of those from the equation.

Won’t Go Away? It Might Be Something Else…

So you thought you had a scum problem, but all the techniques you’ve tried aren’t helping? You may have one of the following problems instead:

  • Calcium scale. Calcium scale deposits are a problem related to hard water. The irony is that hot tubs require hard water in order to be safe for bathers. But if the water is too hard (meaning there’s too much calcium in it), then you’ll start to get calcium scale deposits at the water line and even throughout the filtration system.
  • Mold or mildew. Hot tubs aren’t exempt from mold and mildew formation. When this happens the water may smell funny and you’ll see things floating in it. Pink, black, white, and green mold can all present themselves at any point, as they all thrive in the humid environment of hot tubs. A common place you can find mold is on the inside of the hot tub cover, so don’t forget to clean it when performing regular maintenance on the tub.
  • Algae blooms. Highly contaminated hot tubs are prone to algae blooms. This occurs because there isn’t enough sanitizer in the water to keep it from forming. Algae comes in a few varieties – pink, yellow, black and green – and adding an algaecide in the tub once a week can prevent it from becoming a problem. If you do have an algae bloom, draining and scrubbing out the tub will be your best course of action.

Scum Aint No Fun

Tub crashers are never welcome. Unfortunately, it’s a sad reality that hot tub scum is a common problem in spas.

However, by staying on top of your hot tub’s maintenance, you can keep scum, gunk, and slime from becoming an issue that requires a total water overhaul to fix.

Categories: Hot Tub Care, Hot Tub Problems