How to get rid of mustard algae on your route
Brush every surface, hold free chlorine at about 60% of the cyanuric acid (CYA) reading until the yellow stops coming back, and sanitize every tool that touched the water. Skip the tools and it comes back.
Mustard algae comes back because it survives out of water. Your gear carries it from one pool to the next. The hold level is 60% of CYA, half again the 40% that clears green algae. On a pool at CYA 40, that's 24 ppm.
The chemistry is the easy half. On a 40-pool route, one sick pool can seed several more before you see the pattern. A sanitized net and a smart route order stop it.
Treat it in this order
Mustard isn't the algae a pool gets when it runs out of chlorine. Green algae dies at ordinary shock levels, so treating a pool that has gone green is a different job. Mustard shrugs off normal free chlorine and clings to shaded walls, steps and light niches.
- Brush a patch of wall before you spend any chlorine, and confirm the yellow lifts into a cloud that drifts back down.
- Lower pH to about 7.2, since chlorine kills algae several times faster in slightly acidic water than it does at 8.0.
- Brush every surface, including behind ladders, under steps and in the light niche, because anything you skip reseeds the pool when chlorine drops.
- Raise free chlorine to about 60% of the CYA reading and hold it there. At CYA 40 that's roughly 24 ppm, where a green bloom needs 16.
- Run the filter around the clock for 24-72 hours, because dead algae clouds the water and only the filter clears it.
- Sanitize the net, brush, pole, vacuum head, hoses and test kit, plus any toys or floats the customer owns.
- Keep the level up for a full day after the last yellow is gone, since mustard that looks dead can live on a surface you can't see.
Brush test before you shock
Spring pollen fools techs. It floats in for six to eight weeks, settles in the shallow end, and looks like mustard. A wasted shock costs chemicals and credibility, so brush a patch first. Mustard is stuck to the surface, so it lifts into a yellow cloud and resettles over several minutes. Pollen is already loose and skims off. Silt drops back in about a minute. The readings add a clue. Mustard grows in a pool with normal free chlorine, and pollen has no chemistry story at all. Before you add anything, work out the shock dose for the pool's volume so you aren't guessing from a jug.
| What it is | Where it sits | Brush test | Fix |
|---|---|---|---|
| Mustard algae | Shaded walls, steps, light niche, low-flow corners | Clouds into the water, resettles over several minutes | Shock hold at 60% of CYA, brushing, gear sanitizing |
| Pollen | Water surface and shallow end, seasonal (6-8 weeks in spring) | Disperses or skims off, doesn't cling | Skim, run the filter, clean the skimmer basket |
| Dirt or silt | Pool floor, heaviest in the deep end | Drops back down within about a minute | Vacuum, no chemical needed |
| Dead green algae | Floor, after a recent shock treatment | Lifts and clears through the filter | Keep filtering and vacuum, treatment already worked |
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Your net carries it to the next pool
Mustard survives out of water on damp surfaces. A net left wet in the truck bed between two stops can seed the next pool, and that pool doesn't have to be neglected to catch it. Green algae doesn't travel this way.
On a 40-pool route, yellow dust shows on a shaded Henderson wall on a Thursday. Two weeks later three more pools have it, all serviced after the first on the same run with the same net and brush. That's one outbreak carried four times, and the route order points straight at the first pool.
Treat all four pools, since one left alone restarts the cycle. Soak the gear 10-15 minutes in chlorinated water between stops, or spray it with a strong chlorine solution. That costs a couple of minutes a stop, and a recovery costs hours. Then service the sick pool last until it clears. PoolBoss lets you drag the stop to the end of the day and it stays there. The visit history across the route shows which pool showed it first.
The same wall twice points at circulation
Mustard sets up where flow is weakest and sun is lowest. That means a shaded corner, the area behind a ladder, or the wall farthest from the returns. Chlorine reaches those spots at a fraction of the strength in the middle of the pool. Aim the returns slightly downward and all in one direction so the water turns the whole pool. Then check run time. A residential pool needs 8-12 hours a day for one to two full turnovers, and six hours in July heat leaves it under-filtered whatever the test strip says.
Write the wall on the pool's record. If the same spot lights up a third time after the chemistry and the jet aim are fixed, you have a dead zone. Talk equipment with the customer. That means a return fitting, a jet angle, or a plumber's look at the circulation. Another round of shock won't fix it, and neither will a bottle of algaecide that was never going to fix it.
Prove it held before you close the job
Judge the treatment at the next visit. At dusk, bring free chlorine to the target and write the number down. Read it again before sunrise. A drop of 1 ppm or less means nothing is left eating chlorine. A bigger drop means algae is alive and the hold continues. That reading beats how the wall looks, since mustard can hide for days.
Cloudy water for 24-72 hours after a heavy dose is normal. It's dead algae waiting on the filter, the same reason water turns hazy after a heavy dose after any shock. Dosing again because it still looks bad stretches the haze.
At the follow-up, retest free chlorine, pH and CYA, brush the old wall again, and check the shaded spots. Bring pH back up from 7.2 too. Raising pH without pushing alkalinity up with it takes aeration, not another dose. Then log the readings and the follow-up visit against the pool, on paper or in software. Four pools on one Thursday run, one of them first, shows up in that record, and it's what you show a customer who thinks you skipped their pool.
Key takeaways
- Hold free chlorine at 60% of the CYA reading, 24 ppm at CYA 40, and keep it there a full day past the last yellow.
- Brush a patch before you dose. Mustard clouds up and resettles, pollen skims off, and silt drops in a minute.
- Soak the net, brush, pole and hoses for 10-15 minutes between stops, or one sick pool becomes four.
- Service the sick pool last until it clears, and treat every pool that came after it on the run.
- A yellow patch on the same wall twice means weak flow. Fix jet aim and get run time to 8-12 hours.
- At dusk, bring free chlorine to target and write it down. If it loses more than 1 ppm by sunrise, algae is still alive.
Common questions
How do I keep mustard algae from spreading to the next pool?
Sanitize your gear between stops and put the sick pool last. A 10-15 minute soak in chlorinated water kills what’s on the net. For poles, vacuum heads, hoses and the test kit, keep a spray bottle of strong chlorine solution in the truck. If you have spare gear, give the sick pool its own net and brush until it clears. Tell the customer their toys and floats need the same soak, since those carry it back into the water you just cleaned.
Is mustard algae dangerous to swim in?
The algae itself isn’t a known health risk to swimmers. The chlorine level behind it is the concern, because algae growing means the sanitizer isn’t keeping up, and bacteria can persist under the same conditions. A heavy bloom also hides a swimmer in trouble. Tell the customer to stay out until free chlorine is back at its proper level. Send any rash or illness question to a doctor or the local health department.
Why does mustard algae come back when green algae stays gone?
Green algae dies at ordinary chlorine levels, so once the pool is clear and the ratio to CYA is right, it has no way back. Mustard survives those levels and holds onto surfaces. A skipped shaded spot, a wet net or a customer’s float can each restart it. The treatment needs a higher hold, a longer hold, a full brush and clean gear, all four together. Miss one and the pool looks like the treatment failed, when it was reinfected.
Do I need a special algaecide, or is shock enough?
Chlorine does the killing. Algaecide alone costs $20-$40 a bottle and leaves a mustard bloom where it was, because algaecide prevents growth in clean water and doesn’t clear an established colony. Hold free chlorine at 60% of CYA, brush, and filter nonstop. A yellow-algae product earns a place on a pool that came back twice despite a correct hold. Add a copper or polymer product after the water is clear. If a pool needs algaecide every season, look at circulation and at a maintenance chlorine level set too low for its CYA.
How do I charge for an algae recovery that takes three visits?
Quote it as a separate recovery job before the first treatment. Chemicals run $30-$100 for a residential pool, mostly liquid chlorine, and the job adds two to three visits beyond the regular stop. A flat fee of $150-$350 covers the chemicals and the return trips, depending on pool size and how far the bloom has gone. You can also bill chemicals at cost plus a service-call rate for each extra visit. Don’t absorb it into the weekly rate. If a circulation problem caused it, quote that fix on its own line, since it prevents the next recovery.
Can mustard algae grow in a salt pool?
Yes. A salt system is a chlorine generator, so the same free chlorine reading applies and mustard tolerates it just the same. A salt pool can run low if the cell is scaled, old or set to low output, because the cell makes chlorine at a slow, steady rate. Treat it the same way. Lower pH, brush, and raise free chlorine to the hold level. Put the cell on boost and add liquid chlorine, since a cell can’t reach shock level fast enough alone.
How do I tell a customer this isn’t because I skipped their pool?
Show them the timeline. Mustard grows in pools with normal chlorine, so it doesn’t prove a missed visit or a low dose. Logged readings settle it fast: free chlorine in range at every stop, no gap in service, and the algae showed up anyway. Name the source when you know it, such as a run of pools on one day that showed it in order, or a shaded wall that has been a repeat spot. Then give the plan: the treatment, the gear sanitizing and any circulation fix. A problem with a named cause and a plan goes down easier than one that arrives with a defense.