It shows up the morning after a pool party, or the day after somebody poured in a bottle of algaecide because the water looked a little dull. A band of white suds along the tile line, a little island of foam drifting by the skimmer, and if there’s an attached spa, a full head of foam where the spillover hits. It looks like someone washed a car in the pool. It’s a common enough sight, from pool service routes in Agoura Hills to the coast, that the causes are pretty well mapped out.

Pool foam is rarely dangerous. It’s a sign that something got into the water that shouldn’t have, or that the water is missing something it needs, and it looks a lot worse than it is.

Why pool foam forms in the first place

Water makes bubbles all the time, at the returns, the waterfall, the spa jets. Normally they pop in seconds. Foam happens when something in the water holds the bubble walls together so they last. Soap does exactly that, which is why it’s the easiest comparison.

So the question is never “why are there bubbles.” It’s “what’s keeping them alive.”

Cause one: lotion, sunscreen and everything else people wear

HTH, which makes pool chemicals, puts it plainly: foamy water can come from “a buildup of organic contaminants like body oils, lotions and cosmetics.” Add shampoo and laundry soap left in swimsuits and a busy weekend can do it on its own.

The fix is chlorine and filtration. Raise free chlorine so it can break down the organics, run the pump, and clean the filter if the pressure is up. The prevention is the most ignored sign in pool history: rinse off before you get in. HTH and Angi’s pool foam guide both say the same thing.

Cause two: the wrong algaecide, or too much of it

This is the one people cause on purpose. Angi lists using the wrong algaecide or too much of it as one of the main foam causes, and the product shelf backs that up.

Many algaecides use quaternary ammonium compounds, called quats, which are surfactants. One EPA-registered quat algaecide label lists optional claims like “Non-Foaming Formula” and “Use where foam is a concern,” which tells you the regular kind can foam. BioGuard’s Algae All 60 is sold as a polymeric quat that delivers “the same algae control without the foaming.”

If you’re dosing algaecide to fix a green pool, that’s not what algaecide is for, and real algae removal is mostly chlorine, brushing and filtration. If foam showed up right after a dose, stop adding more, let the chlorine and filter work it out, and switch to a non-foaming product next time.

Cause three: soft water, and why it’s rare here

Low calcium hardness makes water foam more easily. HTH says to keep calcium hardness at 200 to 500 ppm. Angi’s guide uses 200 to 400 ppm. Both agree that too low is a foam problem.

In most of Ventura County, low calcium is not the issue. Ventura County Public Works lists local water hardness at 17 to 53 grains per gallon for the City of Ventura, an average of 28 for Oxnard and 6 to 18 for Camarillo. Using the county’s conversion of 17.1 ppm per grain, Oxnard’s average works out to about 479 ppm of total hardness. That’s the hard water behind the white calcium on pool tile, not foam.

The exception is imported water. The same county article says areas served by Metropolitan Water District supply run up to 5.2 grains per gallon, or about 89 ppm. A pool filled from softer water, or a freshly refilled pool, can test low enough to matter. One calcium test settles it.

Cause four: it’s air, not foam

Sometimes the “foam” is really a stream of fine bubbles coming out of the return jets and collecting on the surface. That’s air getting into the plumbing on the suction side of the pump.

Hayward’s manual says suction leaks “will result in bubbles emanating from the return fittings on the pool wall,” and lists “loose connections on the suction side” as a cause. The usual suspects are a pump lid O-ring that’s dry or dirty, a loose union, or a water level so low the skimmer gulps air. Angi says the skimmer opening should be at least halfway covered by water.

Air leaks don’t go away with chemicals. If the water level is right and the lid is clean and still the returns spit bubbles, that’s a plumbing or valve job, and we cover where those usually hide in what actually breaks on a pool pad.

How to tell which cause you have

Getting rid of pool foam, in order

Test first: free chlorine, pH and calcium hardness. Then fix whichever number is off. If chlorine is low, bring it up and let the filter run. If you just added algaecide, don’t add more of anything for a day. If the returns are bubbling, check the water level and the pump lid before you touch the chemistry.

Old water makes everything here worse. A pool that’s been topped off with hard water for years carries a lot of dissolved material, and at some point the answer is a partial or full drain rather than another bottle of something.

What about anti-foam products?

Defoamers exist and they do knock foam down. They don’t remove whatever caused it. Use one if you have guests coming in an hour. Otherwise, the foam is useful information, and spraying it away just means you’ll see it again next weekend.

Asked at the pad, every week

Is it safe to swim in a foamy pool? The foam isn’t the thing to worry about. The chlorine and pH are. Test before anyone gets in, and if chlorine is low, fix that first.

Why does my pool foam only when the spa spills over? The spillover adds a lot of air at once. If the water has organics or a foaming algaecide in it, that’s where you’ll see it first.

Will shocking the pool get rid of pool foam? If the cause is oils and lotions, raising chlorine helps break them down. It won’t fix low calcium, an algaecide overdose that’s already in there, or an air leak.

Can a salt pool foam? Yes. The salt system makes chlorine, but it doesn’t stop lotion, algaecide or air from getting into the water.

Send us a photo of the foam

A photo of where the foam collects, plus whatever you added to the pool in the last week, tells us most of what we need. Send both through our contact form. On weekly maintenance, the tech emails a report with photos and water readings after every visit, so a calcium or chlorine drift shows up before it turns into suds.

We service Pentair, Jandy, Zodiac, Hayward and Sta-Rite equipment, so if the bubbles turn out to be a pump problem we can handle that too. Or call (805) 586-2055.