No matter how well they take care of their hot tub, every hot tub owner will eventually experience foamy water. Unfortunately, foamy water can also be one of the hardest water quality issues to effectively treat and prevent.

Today, we will look at the various causes of foamy water and look at ways to diagnose, treat, and prevent it from returning. Before we get to that, however, let’s take a quick look at why water foams in the first place.

Why Does Water Foam?

In order to foam, water requires two things; air and a surfactant.

Surfactants are molecules that reduce the surface tension of water; making it easier for oils and water to mix. The combined oils and water then form a thin “skin” on the surface of the water. When air is added to the hot tub water from the jets, the “skin” traps some of that air; forming a bubble. The more surfactants and air that are present in the hot tub, the more bubbles you will get and the longer they will last. Eventually, the bubbles will begin to group together, causing a buildup of foam.

Common Causes Of Foamy Hot Tub Water

The primary reason why foamy hot tub water can be so hard to treat is that no matter how hard you try, there will always be some level of surfactants and oils in your hot tubs water. The key is to keep the amount of each below the point at which they can cause foam buildup.

There are multiple reasons why oils and surfactants can build up in hot tub water; with each of these potential reasons requiring different treatments to fix. The good news is that foamy hot tub water is easily treatable, as long as you know what is causing it. What causes oils and surfactants to build up and cause foamy water in hot tubs? The most common causes are:

  • Soaps and detergents
  • Oils and lotions.
  • Biofilm build-up.
  • Poor water balance.

Soap and Detergents

The most common way that surfactants enter hot tub water is through soap residue on your skin and bathing suits, with the biggest culprit being your bathing suit.

Oils and Lotions

The most common cause of foamy water in hot tubs is a buildup of oils and lotions in the water. Every time you use the hot tub, you can potentially add a variety of oils and lotions to the water, including:

  • Body oils.
  • Deodorant.
  • Beauty products.
  • Body lotion.

Although they don’t really have any negative effects in low amounts, over time these substances can build up in the water, especially on or near the surface of the water. If left untreated, the oils will build up to a level that will cause foam.


Biofilm is defined as “any group of bacteria and other micro-organisms that stick themselves to a surface which is in regular contact with water”. If not treated biofilm will eventually stick itself onto the surfaces in the hot tub, most commonly in the plumbing. Once stuck on it will grow a protective layer of slime and begin to grow.

This protective layer is very hard for sanitizers like chlorine or bromine to effectively treat. As biofilm continues to grow it will use up more and more of your sanitizer and leave less and less to break down surfactants and other substances that will eventually cause the water to foam.

Poor Water Balance

Poor water balance – especially water with low calcium hardness – is another common cause of foamy hot tub water. While water balance alone will not cause foamy water, poorly balanced water reduces the surface tension of water in the same way that a buildup of surfactants will; amplifying other potential issues.

Read More: Brady’s Hot Tub Maintenance Guide

Diagnosing Foamy Hot Tub Water

With so many causes of foamy hot tub water it can be tough to treat. The key to effective treatment is first properly diagnosing the cause of the foam. Start by taking your water in for professional testing. If your water is out of balance there is a good chance that your issue is at least partially due to poor water balance.

If the balance of the water is fine but your sanitizer levels are low, the issue could be due to a buildup of biofilm. Add more sanitizer and test your water again the next day (your home test kit will be fine for this second test). If the sanitizer level is low again, you likely have a biofilm buildup in the hot tub.

If your water is balanced and holding a good sanitizer residual, the issue could be a buildup of oils and lotions. A good sign that a buildup of oils and lotions might be the cause is if you get an oily feeling “ring” on the shell of the hot tub around the water’s surface.

If you still can’t find an obvious issue then a buildup of surfactants is the most likely answer.

Treating Foamy Hot Tub Water

Now that you have an idea of the cause, treating your foamy hot tub water should be much easier.

To properly treat foamy water, first rebalance your water (if needed). If the problem isn’t completely fixed by rebalancing the water, you will need to remove the buildup of oils, lotions, and/or soaps that are causing the issue. This can be done by simply adding an oil-absorbing sponge to the water. This sponge will slowly absorb any oils, soaps, and lotions on the surface of the water. Over the course of a few days, you should notice the foam recede and finally stop. If the foam doesn’t completely subside within a week, you can add a 2nd sponge to speed up the process, or simply drain and refill the hot tub.

If you suspect your foamy water issue is due to biofilm, draining and cleaning the hot tub pipes to remove the biofilm buildup is the only answer. Contact us for more information.

Note: While there are “anti-foam” chemicals that you can add to the water, these are simply designed to temporarily break the bond between the water and the surfactants. If you don’t treat the water further, the foam will return once the anti-foam chemical has been used up (typically within a day).

Preventing Foamy Hot Tub Water

The easiest way to prevent your hot tub water from foaming is to reduce the amount of oils, lotions, and soaps that you introduce into the water. The easiest way to do this is to have a quick shower before using the hot tub to remove most of the skincare products, makeup, deodorant or other personal care products that you might have on your skin.

You can also greatly reduce the amount of laundry detergent and fabric softener that gets introduced to the water by washing your bathing suits separately using less detergent and giving them an extra rinse cycle before drying them.

If you already do these things, consider using an enzyme-based chemical such as Spa Perfect or Spa Synergy Clean. Enzymes naturally break down non-living organic contaminants such as body oils, cosmetics, and suntan lotions and are therefore another great way to reduce the buildup of oils and surfactants.

No matter how hard you try though, surfactants will slowly build up in the water. If your hot tub water is almost ready to change and starting to get a little foamy, regularly adding a small amount of anti-foaming chemicals to the water weekly can help eliminate the foam until it is time to refill the hot tub.

Read More: Hot Tub 101 – The Basics Of Hot Tub Ownership


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