Discount Pool Supply › Guides › Water chemistry
It brushes off easily and comes back in a week. Here is why, and the full routine that actually gets rid of it.
Mustard algae looks like yellow or brown dust on the shady walls and steps. It brushes away in a cloud, then comes back in the same spots a few days later.
Mustard algae tolerates normal chlorine levels better than green algae, and it hides on anything that went into the pool: brushes, toys, swimsuits, and cleaner hoses. Kill it in the water and it returns from the gear.
Brush every surface. Raise chlorine well above normal and hold it there for a day or more, with the pump running around the clock. Test and adjust pH first, because chlorine works poorly when pH runs high. Clean the filter afterward, since dead algae ends up in it.
Soak brushes, toys, and cleaner parts in chlorinated water, and wash swimsuits that were in the pool. Skipping this step is the most common reason it returns.
Steady chlorine and a stabilizer level in range matter more than big doses once in a while. Bring a sample to either store; we test for phosphates and stabilizer as well as chlorine and pH, and write out the plan.