Real-life case: the mustard algae that came back every fortnight
- The situation: mustard algae treated three times… back three times, at 15-day intervals, in the same spot.
- The first two treatments failed for the classic reasons: underdosed shock, then undisinfected equipment.
- The final culprit: the robot — and its never-treated filter, reseeding the pool on every run.
- Golden anti-mustard rule: treat the pool AND everything that touches the water, robot included.
First appearance: a yellow-beige deposit on the step on the shady side, late June. A pass of the brush, a little extra chlorine — gone. A fortnight later: it’s back. Same spot, same mustard-coloured dust billowing up in a cloud as the brush goes by. A more serious treatment this time. A fortnight later: back again. When a pool problem returns with the regularity of a subscription, it’s no longer bad luck — one link in the treatment is failing. Here’s the investigation, failure by failure.
Failure #1: the shock that wasn’t one
The first real treatment was a classic shock, dosed “as for green water”. Calibre error: mustard algae tolerates the concentrations that kill green algae. A standard shock makes it fade and retreat — it moves back in as soon as the level drops. That’s its signature, and the reason the app has a separate mustard protocol: a significantly boosted chlorination, calculated on the volume, and held long enough. Lesson one: against mustard, half measures are a subscription to relapse.
Failure #2: the pool treated, the equipment forgotten
Second round, this time at the right calibre, the app’s plan followed… except for one step, judged incidental: disinfecting the equipment. A fortnight later, the yellow deposit was back. Mustard algae has a superpower: it clings to everything — brush, net, hose, swimsuits. Treating the pool without treating what touches it is like disinfecting a room while keeping your dirty shoes on. Lesson two: the “incidental” step of the plan wasn’t — none of them are.
The final investigation: the stowaway
Third treatment: full calibre, equipment soaked in the pool during the boosted chlorination, swimsuits through the machine. And yet, the nagging worry: what had survived twice? The answer was sleeping in the plant room: the robot — and above all its filter cartridge, rinsed with clean water after every cycle, never disinfected. A perfect algae reservoir: damp, dark, and released back into the pool on every run, while stirring up the dead zones on the floor.
This time: cartridge soaked in a chlorinated solution, shell and brushes treated, dried completely in the sun — while the pool got its protocol. Result: not a single relapse since. The “mystery” of the fifteen days was simply the rhythm of the robot’s runs.
What this case teaches
Mustard algae isn’t “treated”, it’s eradicated — and eradication is a matter of complete method: sufficient concentration, sufficient duration, and everything that touches the water, robot and crannies included. That’s exactly what the app’s mustard plan rolls out: every step in order, none judged incidental, doses calculated for your pool. And if it still comes back despite all that, the app runs the investigation with you: stabiliser too high, forgotten equipment — the list of suspects is short when the protocol is square.
Frequently asked questions
Why does mustard algae always come back?
Two causes dominate: a treatment at insufficient concentration (it tolerates standard chlorine levels) and recontamination via equipment — brush, net, robot, swimsuits. As long as the reservoir survives somewhere, the pool reseeds itself.
How do you disinfect a pool robot?
Filter/cartridge removed and soaked in a chlorinated solution, shell and brushes rinsed with strongly chlorinated water (or run in the pool during the boosted chlorination), then dried completely in the sun. The robot's filter is hiding place number one.
How can you be sure it's mustard algae and not pollen or sand?
The brush test: mustard algae billows up in a yellow cloud as the brush passes and settles back in the same spot within hours. Pollen tends to float on the surface, sand doesn't disperse in a cloud.