The quick answer: foam means surfactants (soap-like substances) have built up in the water. Soap, shampoo, lotion, cosmetics and laundry detergent in swimsuits are all surfactants, and the jets whip them into bubbles.
- Short term: shock to break the organics down, and clean the filter.
- For good: cut what's going in, keep sanitizer up, and drain when the water's dissolved solids are high.
- Defoamer only hides the bubbles while you do that.
What causes hot tub foam
- Personal products. Body lotion, sunscreen, deodorant, makeup, hair products and the residue of shower gel all rinse off in a tub.
- Laundry detergent in swimsuits and towels. Swimsuits washed in a machine carry detergent into the water. This is one of the biggest causes.
- Old water. Every soak and every chemical dose adds dissolved solids. Past a point, the water foams no matter what you do. See how often to drain a hot tub.
- Low sanitizer. Organic waste isn't oxidized fast enough and accumulates.
- A dirty filter. Oils trapped in the cartridge keep leaking back into the water.
- Soft water. Very low calcium hardness makes water foam more easily. Many manufacturers recommend a calcium range (often roughly 100–200 ppm); check yours.
- Some chemicals. Certain algaecides, clarifiers and cheap "all-in-one" products can foam. Household detergents used to clean the shell are a common cause.
How to get rid of foam: step by step
- Test sanitizer, pH and alkalinity. Bring sanitizer back into range: chlorine 3–5 ppm, bromine 4–6 ppm.
- Shock the tub. Shock oxidizes organic contaminants, including many foam-makers:
- chlorine users: chlorine shock;
- bromine users: MPS. Run the jets with the cover open.
- Clean the filter. Rinse it, then soak it in a filter cleaner to strip oils. Use a spare cartridge while one dries, if you have one.
- Skim off the foam, or use a little defoamer to collapse it so you can see the water. Defoamer is a temporary fix. Overusing it can leave a residue and cloud the water.
- Check calcium hardness. If it's very low, raise it to the manufacturer's range with calcium increaser (calcium chloride), using the calculator.
- If it returns within a few days, drain and refill. Run a line flush product first, so biofilm and residue leave the plumbing too.
Foam that only appears with the jets on
That's normal for foamy water. Jets and blowers add air, and surfactants trap it as bubbles. Turning the jets off hides it but doesn't fix it. Follow the steps above.
If the water is clear and only big, fast-popping bubbles appear, it's probably just air from the jets, which isn't foam.
What the foam looks like
| Foam type | Usually means |
|---|---|
| White, fluffy, pops slowly | Soap, detergent and lotion residue |
| Greasy, yellowish film with foam | Body oils and cosmetics; heavy organic load |
| Brown or dirty foam | Organics plus debris; possibly metals or a very old filter |
| Foam right after adding a chemical | That product contains surfactants. Check its label. |
Prevent it coming back
- Rinse off before soaking. Plain water is enough to remove lotion and hair products.
- Rinse swimsuits in plain water, and don't machine-wash them with detergent, or rinse them twice if you do.
- Don't clean the shell with household cleaners. Use a non-foaming spa surface cleaner, and rinse well before refilling.
- Dose sanitizer after soaks, and shock weekly with regular use.
- Rinse the filter weekly and deep-clean it monthly.
- Drain on schedule. Smaller tubs (especially inflatables) need it more often.
- Keep calcium in your tub's recommended range if you have very soft water.
Using defoamer the right way
Defoamer is useful when guests are about to arrive and you need the surface clear. Use it sparingly:
- Start with a small amount (a few drops to a capful, per the label) on the foam with the jets running. Add more only if needed.
- Don't use it as a routine additive. Defoamers are typically silicone-based. Overuse can coat the filter, leave a greasy film and make cloudiness worse.
- Pair it with a fix. If you reach for defoamer more than once a week, the water is telling you it's due for a shock, a filter clean or a refill.
Foam vs scum line
A scum line is a greasy ring at the waterline, made of body oils and lotions that float. It often comes with foam because both have the same source. Wipe it off with a non-foaming spa surface cleaner or an absorbent "scum ball" in the skimmer area, never household detergent. A recurring scum line is another sign the organic load is high and the water is ageing.
Inflatable tubs foam more easily
Small volume and a small filter mean contaminants concentrate quickly. Rinsing before every soak and cleaning the filter often make a big difference, and drain intervals are usually shorter than for acrylic tubs. See the inflatable hot tub chemicals guide.
Track foam in Spa Calc
Add a "foam" note in Spa Calc whenever you see it. Your log will show whether it follows heavy soaks, low sanitizer or old water, and the app tells you to drain early when stabilizer or dissolved solids get too high. Every dose is calculated for your tub's volume and the products you own.
One hot tub? Every feature, free. No account, no ads, and iCloud backup is included.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.