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One of the most common water quality issues that hot tub owners experience is foamy water. For many, it can also be one of the hardest to effectively treat. Today, we will look at the various causes of foamy water. We will also look at ways to properly diagnose, treat, and prevent foamy water from returning.

Why Does Hot Tub Water Foam?

Before we get into the causes of foamy hot tub water, lets first quickly take a look at why hot tub water foams in the first place.

In order to foam, water requires two things.

  • Air.
  • A Surfactant.

Surfactants are molecules that reduce the surface tension of water; making it easier for oils and water to mix. When air is added to the water, the surfactants and oils form a thin “skin” around the surface of the water; 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 main reason why foamy hot tub water can be so hard to treat is that there are multiple reasons why water can become foamy; with each of these potential reasons requiring different treatments to solve. What caused your water to foam in the past also might not be the reason why your water is currently foamy, leading to further confusion and frustration with hot tub owners.

The good news is that foamy hot tub water is easily treatable, as long as you know what is causing it. What causes foamy water in hot tubs? The most common causes are:

  • A buildup of oils, soaps, and lotions.
  • Poor water balance.
  • Biofilm build-up.

Oils, Soaps, and Lotions

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

  • Body oils.
  • Deodorant.
  • Laundry detergent & fabric softener.
  • Beauty products.
  • Body lotion.

All of these substances are surfactants. 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. Once they build up to a critical level, the surface tension of the water will be reduced to a point where it will mix with oils and other surfactants to create 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 will reduce the surface tension of water; amplifying other potential issues.

Biofilm

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 grow on the surfaces in the hot tub, most commonly in the plumbing. Like poor water balance, biofilm itself does not directly cause foamy water. It does, however, rapidly use up the sanitizer (choline or bromine) residual in the water which normally helps to break down the surfactants that cause foamy water.

With no sanitizer in the water, these substances build up much more quickly than normal and can rapidly lead to a foaming issue.

Diagnosing Foamy Hot Tub Water

So now that we know the common causes of foamy water, the question becomes how to tell which one is causing your issue. The first step is to take your water in for professional testing. If your water is out of balance there is a decent chance that your issue is at least partially due to your 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 this time). 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 is likely a buildup of oils, soaps, and lotions. Another sign that oils and lotions might be an issue is if you get an oily feeling “ring” on the shell of the hot tub around the water’s surface.

Treating Foamy Hot Tub Water

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).

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, soaps, and lotions 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 surfactants in 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, follow this guide to treating and preventing biofilm buildup.

Preventing Foamy Hot Tub Water

The easiest way to prevent your hot tub water from foaming is to reduce the amount of surfactants that you introduce into the water. The easiest way to do that 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.

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.