How do you get rid of mustard (yellow) algae in a pool?

Mustard, or yellow, algae tolerates the chlorine that kills green algae, so it takes a sustained, aggressive treatment. Brush every surface to break its protective film, balance the water to pH 7.2 with CYA down at 20 to 40 ppm, then shock free chlorine to 15 to 20 ppm and add a mustard algaecide. Clean the filter hard, soak all pool gear to kill reinfection, and hold elevated chlorine a full 72 hours before declaring victory.

How is mustard algae different from green algae?

Green algae floats and turns the water cloudy. Mustard algae attaches to surfaces instead, walls, steps, light niches, and ladders, and forms a thin yellowish film that looks like pollen or sand. It brushes off easily but returns fast, and it clings to shady spots where it resists normal chlorine.

What free chlorine level do I need to kill mustard algae?

Push free chlorine to 15 to 20 ppm and hold it long enough to penetrate the biofilm. For a 15,000-gallon pool that is roughly 3 pounds of cal-hypo at 65 percent or about 3 gallons of liquid chlorine. Lowering CYA to 20 to 40 ppm first means the chlorine works harder.

Why does mustard algae keep coming back?

It reintroduces itself through contaminated gear. Mustard algae lives on pool toys, float noodles, and cleaning equipment brought from other pools, so if you skip treating them it comes right back. Soak all gear in high-chlorine water or diluted bleach, and don't under-treat, which is the other main reason it returns.

What should I do with the water after the algae clears?

Once the water clears, hold free chlorine at 5 to 7 ppm for two weeks to guarantee a complete kill. If mustard algae has been a recurring problem, add a weekly dose of a non-foaming, non-quat algaecide as maintenance. Don't stop at the first clear day, run elevated chlorine a full 72 hours and deep-clean the filter first.

Do I really need to brush before shocking for mustard algae?

Yes. Brushing walls, floor, steps, light niches, and ladders breaks the algae's protective film so the chlorine can actually reach it. Then keep brushing every few hours while chlorine is elevated, run the filter continuously, and backwash or clean it after 24 hours because mustard algae clogs filter media fast.

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Quick answer

Mustard algae is chlorine-tolerant — it shrugs off the 1–3 ppm free chlorine that kills green algae, so a normal shock won't touch it.
Drive free chlorine to 15–20 ppm and hold it there — roughly 3–5× what green algae needs — to break through the protective coating that shields the algae.
Brush every surface first — walls, floor, steps, light niches, and ladders — because scrubbing breaks the mucilaginous film so chlorine can reach the algae.
Balance for treatment before you shock — pH 7.2, TA 80–100, and CYA down at 20–40 ppm so chlorine works as hard as possible.
Treat every piece of gear — toys, noodles, and cleaning tools reinfect the pool, so soak them in high-chlorine water or diluted bleach or it comes right back.
Don't quit at the first clear day — run elevated chlorine a full 72 hours, deep-clean the filter, then hold FC at 5–7 ppm for two weeks.

Mustard algae — also called yellow algae — is the most stubborn algae species Florida pool owners encounter. It looks like pollen or sand on the pool walls and floor, clings to shady spots, and resists normal chlorine levels. Clearing it requires a sustained, aggressive treatment.

What mustard algae is

Phaeophyta (yellow-green algae) is chlorine-tolerant. Unlike green algae that floats and turns water cloudy, mustard algae attaches to surfaces — walls, steps, light niches, ladders — and forms a thin, yellowish film that brushes off easily but returns fast. It can also live on pool equipment and accessories brought from other pools.

Why normal chlorine doesn't work

Mustard algae has a protective mucilaginous coating that shields it from chlorine at normal levels (1–3 ppm). You need to drive free chlorine to 15–20 ppmand maintain it long enough to penetrate the biofilm — that's 3–5× what green algae requires.

Treatment protocol

Brush all surfaces thoroughly— scrub walls, floor, steps, light niches, and ladders. This breaks the algae's protective film.
Test and balance water — target pH 7.2 (lower end increases chlorine efficacy), TA 80–100, CYA at 20–40 ppm for treatment (lower CYA means chlorine works harder).
Shock heavily — add cal-hypo or liquid chlorine to reach 15–20 ppm FC. For 15,000 gallons this means roughly 3 lbs cal-hypo (65%) or 3 gallons liquid chlorine.
Add a mustard algaecide— “Yellow Out” or similar sodium bromide + shock combo products work well. Follow product directions exactly.
Run pump and brush again — brush every few hours while chlorine is elevated. Run filter continuously.
Clean filter aggressively — backwash or clean after 24 hours. Mustard algae clogs filter media fast.
Treat all equipment — soak pool toys, float noodles, and cleaning equipment in high-chlorine water or diluted bleach. Mustard algae re-introduces itself through contaminated gear.

After treatment

Once water clears, maintain FC at 5–7 ppm for two weeks to ensure complete kill. Consider a weekly algaecide dose of a non-foaming, non-quat product as maintenance if mustard algae has been a recurring issue in your pool.

Mustard algae always comes back when people under-treat. Don't stop at the first clear day — run elevated chlorine for a full 72 hours and deep-clean the filter before declaring victory.

Frequently asked questions

Mustard algae carries a protective mucilaginous coating that shields it from chlorine at normal levels of 1 to 3 ppm. To penetrate that biofilm you have to drive free chlorine to 15 to 20 ppm and hold it there, which is 3 to 5 times what green algae requires.