Foamy hot tub water: why it happens and how to stop it

Hot tub foam is caused by things dissolved in the water that act like soap: laundry detergent in swimwear, body lotion, shampoo, cosmetics, low calcium hardness, or water that is simply old. To get rid of it, test and balance the water, shock it, clean the filter, and raise calcium hardness to 150–250 ppm. If the water is over 3 months old, change it.

Why hot tubs foam

Foam needs three things: water, air, and a surfactant. A surfactant is anything that makes water "soapier." Jets supply the air. The surfactants come from bathers and products.

Source	Clue	Fix
Laundry detergent and fabric softener in swimwear	Foam appears when certain people get in	Rinse suits in plain water. Skip the washing machine.
Lotions, deodorant, hair products, makeup	Oily film at the waterline	Quick shower before soaking
Soft water	Calcium hardness under 150 ppm	Add calcium hardness increaser per the label
Old water with high dissolved solids	Water over 3–4 months old, dull, hard to balance	Drain and refill
Low sanitizer or biofilm	Foam plus cloudiness or a musty smell	Shock. Line flush at next drain.
Cheap or overdosed chemicals	Foam right after adding an algaecide or clarifier	Use spa-labeled products at label doses
Filter cleaned with household soap	Foam right after a filter cleaning	Rinse thoroughly. Use a spa filter cleaner.

How to get rid of foam

1. Test the water. Check sanitizer, pH, alkalinity, and calcium hardness.
2. Balance alkalinity and pH. Aim for 80–120 ppm and pH 7.2–7.8.
3. Raise calcium hardness if it is under 150 ppm. Follow the product label for your tub's volume. See the calcium hardness guide.
4. Shock the tub. A chlorine shock with the cover off oxidizes oils and lotions. See how to shock.
5. Clean the filter. Power off at the breaker, hose the pleats, and soak in a spa filter cleaner. Rinse until no suds appear. See how to clean a hot tub filter.
6. Skim off the foam with a net or a cup while the jets run. You are physically removing the surfactant.
7. Retest after 24 hours. If foam returns as soon as the jets start, the water is saturated and needs changing.

The quick fix: defoamer

A spa defoamer knocks foam flat in seconds. It is fine for a night when guests are over. It does nothing about the cause, and it adds more product to the water. If you reach for it more than once or twice between water changes, work through the steps above.

When to drain

Drain and refill if:

• The water is more than 3 to 4 months old.
• Foam is thick and comes back within minutes of skimming.
• The foam is off-white or tan and smells, which points to heavy organic load.
• Someone used the tub with soap, bubble bath, or oils. No chemical removes those.

Before draining a persistently foamy tub, use a line flush product so residue in the pipes does not foul the new water. Then follow how to drain a hot tub.

Is it foam or just bubbles?

Clean water makes bubbles that pop within a few seconds of turning the jets off. Foam holds its shape, piles up, and lingers for a minute or more. A few bubbles around the jets with the air controls open is normal and needs no treatment.

Preventing foam

• Rinse before you soak. A 30-second shower removes most lotion, deodorant, and hair product.
• Keep hair up and out of the water.
• Have tub-only swimwear. Rinse it in plain water and hang it to dry.
• Keep calcium hardness at 150–250 ppm. Check after every refill, especially if your house has a water softener.
• Shock weekly and after gatherings.
• Clean the filter on schedule and never with dish soap or laundry detergent.
• No drinks in the water. Beer and sugary drinks foam.
• Change the water every 3 to 4 months.

A note on water softeners

If your hose bib is on softened water, your fill will have very low calcium and will foam easily. Fill from an unsoftened tap if you have one, or add calcium increaser after filling. Soft water also slowly corrodes heater elements and pump seals, so this is worth fixing for more than looks.