Foamy Hot Tub Water: Causes and How to Get Rid of It

What causes foam

Water by itself doesn't foam. Something dissolved in it has to hold the bubbles together, and the jets and air blowers whip the water into foam. The usual sources:

Detergent in swimsuits. Suits washed in a machine carry laundry soap into the tub. Residue can stay in the fabric after a normal wash.
Personal care products: lotion, sunscreen, deodorant, makeup, hair spray, and shampoo residue.
Body oils and sweat. Every soak adds some. Hot water and many bathers in a small amount of water make it build up fast.
Old water. Total dissolved solids (TDS) build up from chemicals, evaporation, and bathers. The more there is, the more stable the bubbles.
Some products. Certain algaecides and scented spa additives foam when agitated. Check the label if foam started right after adding something new.

White, fluffy foam that disappears within minutes of turning the jets off is mostly a nuisance. Thick, gray or brown foam that stays on the surface, with a scum line, means a lot of organic waste: treat it as a sanitizer problem too.

Quick fix vs real fix

Two ways to deal with foam
Question	Quick fix: anti-foam	Real fix: remove the cause
What it is	A defoamer that breaks surface tension	Shock, a filter cleaning, and fresh water if needed
Works in	Minutes	A day, or one drain
Lasts	Until the next soak or two	Until the waste builds up again
Downside	Adds to what the filter has to catch and doesn't fix the cause	Takes time; draining uses water

The real fix, in order:

Test and balance: sanitizer, pH 7.4–7.6, and alkalinity 80–120 ppm.
Shock with chlorine to oxidize the oils and soaps. Use the calculator on the hot tub shock page. Leave the cover off with the jets running.
Clean the filter. Oils coat the pleats, and a hose rinse alone won't remove them. Soak the filter in a filter cleaner, or swap in a spare.
If foam comes back within a day or two of shocking, the water is probably too old to fix. Drain and refill.

Calcium and TDS check

Calcium hardness. Very soft water (low calcium hardness) is often blamed for foam. We could not confirm that link with a primary source, but it costs little to check: the target for hot tubs is 150–250 ppm. If you're below about 100 ppm, raising it is reasonable for other reasons too, since soft, hot water is harder on metal parts. In a 400-gallon tub, about 109 g (3.8 oz) of 77% calcium chloride adds 50 ppm; dissolve it in a bucket of tub water first.

TDS (total dissolved solids). The industry guideline cited by PHTA is to replace water when TDS has risen more than 1,500 ppm above your fill water. You need a TDS meter or test to know. Measure your fresh fill once, write it down, and compare later. Without a meter, the age of the water and the number of soaks since the last fill are a reasonable substitute.

Drain-and-refill interval

The Pool & Hot Tub Alliance suggests draining every 3–4 months, or as your manufacturer recommends. For a tub that gets heavy use, a widely repeated rule of thumb divides your tub's gallons by 3, then by the number of people who soak on an average day, to get the days between water changes. We could not confirm where the rule comes from, so treat it as a starting point and drain sooner if foam or cloudiness returns quickly.

Days between water changes by the rule of thumb (gallons ÷ 3 ÷ soakers per day)
Tub size	1 soaker a day	2 soakers a day	4 soakers a day
200 gal	67 days	33 days	17 days
300 gal	100 days	50 days	25 days
400 gal	133 days	67 days	33 days
500 gal	167 days	83 days	42 days

For your own numbers, use the hot tub water change calculator. Small inflatable tubs reach their limit fastest because they hold the least water.

Prevent it

Shower without soap before getting in, or at least rinse off lotion and deodorant.
Rinse swimsuits in plain water after use, and keep "hot tub suits" out of the washing machine, or wash them without detergent.
Keep hair products and makeup out: tie hair up and remove makeup first.
Shock weekly and after busy sessions. Clean the filter on schedule. The weekly hot tub routine has the timing.
Skip scented additives and use only the products you need.

Foam and cloudiness often come together. If the water is also hazy, see cloudy hot tub water fixes. Pool owners with the same problem can read foam in a pool.

Safety

Don't soak if sanitizer is below 3 ppm chlorine or 4 ppm bromine, foam or not.
Never mix anti-foam, shock, or filter cleaner with each other. Rinse filters thoroughly after using a cleaner so it doesn't go back into the tub.
If someone swallows spa chemicals or gets them in their eyes, call Poison Control at 1-800-222-1222.
Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.

FAQ

Why is my hot tub foaming when jets are on?

The jets whip air into water that contains soaps, lotions, or oils, and those hold the bubbles together. Turn the jets off and the foam usually collapses. Shock, clean the filter, and if it keeps happening, change the water.

Is hot tub foam dangerous?

The foam itself isn't, but it tells you there is waste in the water that uses up sanitizer. Test before soaking: if chlorine or bromine is low, fix that first. Thick, dirty-looking foam calls for a shock.

Does shock get rid of hot tub foam?

Often, yes, when the cause is fresh organic waste like lotions and oils. If the water is old and full of dissolved solids, shock helps only briefly, and a drain and refill is the real fix.

Sources

PHTA / AQUA Tech Notes (TDS guideline, spa ranges from ANSI/APSP-11): PDF
Pool & Hot Tub Alliance (PHTA), Maintaining your hot tub (drain every 3–4 months): phta.org
Taylor Technologies, K-1005 instruction manual (calcium hardness ranges): PDF
CDC, Home pool and hot tub water treatment and testing (sanitizer minimums): cdc.gov