Chlorine OK But Recurring Mustard Algae Only Shallow End Decision Tree

Why this matters

Mustard algae (yellow algae) that keeps coming back in the shallow end while your chlorine tests read fine is a classic "the numbers say healthy, the pool says sick" disagreement. The customer sees a recurring tan-yellow dust on the shallow slope and steps; you see a normal free chlorine (FC) reading and assume sanitation is fine. Both are true, which is the trap. Mustard algae is chlorine-tolerant, lives where circulation is weakest and sunlight is strongest, and reseeds from anything that touched the pool. Solve only the chemistry and it returns; solve only the brushing and it returns. This tree separates the FC-versus-CYA ratio problem, the dead-spot circulation problem, and the reinfection-source problem so you fix the actual driver.

Symptom presentation

A powdery, mustard-to-tan film on the shallow-end floor, steps, and behind ladders. It brushes away easily into a cloud and resettles within a day or two, always in the same low-flow, high-sun zone. FC reads in the normal band, sometimes even on target. CC may be slightly elevated. The deep end and well-circulated returns stay clean. Often the customer mentions it came back after they thought it was gone, and it tends to recur in warm, sunny months.

Quick checks

• Test FC against CYA, not in isolation. A "normal" FC of 2 to 3 ppm is far too low if CYA is high; mustard algae persists when the maintenance FC/CYA ratio sits below roughly 5 percent and is reliably killed only when shocked up toward and past the 10-to-12 percent range for the duration of treatment.
• Map circulation in the shallow end: where do return jets actually reach? Aim eyebrow and wall returns and run a dye or clean-water test to see if the dead spot is hydraulic rather than chemical.
• Check for a persistent dead zone behind steps, under a tanning ledge, or around a recessed light where flow stalls and the shallow water is warmest and sunniest, the conditions mustard algae prefers.
• Identify reinfection vectors: pool toys, floats, swimwear, cleaning brush, the leaf rake, and the automatic cleaner that all carry mustard algae spores between visits and reseed the same zone after every treatment.
• Confirm CYA is not so high that effective FC is suppressed even at a normal absolute reading; over-stabilized water (CYA well above 80 ppm) makes the required kill-level FC impractical to hold.
• Brush the affected area and watch how fast it resettles; a film back within a day points at an active dead-spot colony, not just loose debris.

Isolation tree

1. Compute the FC/CYA ratio. Is FC below about 7.5 percent of CYA? If yes, effective sanitizer is too weak for a chlorine-tolerant algae regardless of the "normal" absolute FC. Raise the ratio (see remediation) and re-evaluate. If the ratio is adequate, continue.
2. Is the affected area a clear circulation dead spot (poor return reach, stagnant corner, behind steps)? If yes, the algae colonizes where chlorinated water does not flow. Improve circulation and re-check. If flow reaches the area, continue.
3. Are reinfection vectors uncontrolled (untreated toys, floats, brush, cleaner, swimwear)? Mustard algae survives on these and reseeds the same shallow zone after each treatment. Sanitize the vectors and re-check. If vectors are controlled, continue.
4. Is CYA so high that even an elevated FC cannot hold an effective ratio without unrealistic dosing? If yes, partial drain/dilute to lower CYA so the ratio is achievable. If CYA is reasonable, continue.
5. With ratio, circulation, and vectors all addressed, does it still recur? Suspect a biofilm reservoir in the shallow plumbing or a phosphate-rich environment feeding rapid regrowth; treat the surface mechanically and chemically together.

Confirming diagnosis

Confirm by treating the whole system at once and watching whether the shallow end stays clean through a full cycle. The diagnosis is confirmed when a single coordinated pass (raise FC/CYA ratio, brush the dead spot hard, sanitize every vector, fix circulation) clears it and it does not return within two service cycles. If it returns despite all three, the remaining driver is a circulation dead zone you did not fully correct or a reinfection vector you missed (most commonly the automatic cleaner or a favorite float left in the pool).

Remediation

• Raise the effective sanitizer: shock to an elevated FC appropriate to the CYA level so the FC/CYA ratio reaches the mustard-algae kill range, and hold it there, not a single dose.
• Brush aggressively and daily during treatment, focusing the shallow slope, steps, and behind ladders; brushing breaks the protective layer so chlorine can reach the cells.
• Sanitize every vector: soak or chlorine-rinse pool toys, floats, brushes, the leaf rake, the automatic cleaner, and run swimwear through a wash; reintroduce only after the pool clears.
• Fix circulation: aim returns to sweep the dead spot, run the pump longer during treatment, and clear any clogged eyeball fitting.
• Lower CYA by partial drain/refill if the ratio cannot otherwise be reached.
• After clearing, hold the FC/CYA ratio at the maintenance floor and re-brush the former hot spot for a few visits to break the recurrence pattern. Add a phosphate check if the regrowth is unusually fast; a phosphate-rich shallow zone gives the surviving cells a head start.
• Treat the cleaner and filter as part of the pool: run the cleaner through the shock, and backwash or clean the filter afterward so trapped spores do not pump back into the shallow end on the next cycle.

References

• PHTA/APSP-11, "Standard for Water Quality in Public and Residential Pools and Spas" (FC, CYA, and effective-chlorine relationships).
• CDC, "Model Aquatic Health Code (MAHC) Code and Annex" (disinfection, combined chlorine, and circulation/turnover requirements).
• Taylor Technologies, "Pool & Spa Water Chemistry" reference (cyanuric-acid effect on chlorine effectiveness and algae control).
• Orenda / industry technical guidance on FC/CYA ratio targets for chlorine-tolerant algae (mustard/yellow algae remediation thresholds).