Mustard algae, often mistaken for sand, dirt, or yellow staining on pool surfaces, is a highly chlorine-resistant form of algae that presents a unique challenge to pool sanitation. This organism develops protective compounds that shield it from standard chlorine levels. Unlike its free-floating green counterpart, mustard algae stubbornly clings to walls, floors, steps, and pool accessories, only to reappear quickly after brushing. A typical saltwater pool’s chlorine output is generally unable to eradicate this persistent pest, requiring an aggressive and specific multi-step treatment protocol.
Confirming the Problem and Preparing the Pool
Identifying mustard algae is confirmed by its characteristic yellow or brownish-yellow hue; it brushes off easily but settles quickly and returns almost immediately. Before chemical treatment, physical preparation and water balancing are necessary to maximize the effectiveness of the algaecide and shock. Remove all accessories from the pool, including ladders, floats, toys, and automatic cleaners, as these items harbor spores and can easily reinfect the water later.
All affected pool surfaces, including the walls, floor, and steps, require comprehensive manual brushing to loosen the algae from the surface pores and expose it to the forthcoming chemical application. After this initial brushing, the water chemistry must be tested and adjusted to ensure the sanitizers work at their peak potential. High pH levels, common in saltwater pools due to the chlorine generation process, significantly reduce chlorine’s effectiveness.
The pH level should be lowered to a range of 7.2 to 7.4, with alkalinity adjusted to between 80 and 120 parts per million (ppm). Balancing these levels before adding the shock ensures the chlorine is not wasted fighting the algae while hampered by poor water conditions.
The Aggressive Chemical Treatment Protocol
Eradicating mustard algae requires super-chlorination, often referred to as the SLAM method, where chlorine levels are temporarily raised far beyond the normal operating range. For infestation, achieve a free chlorine level three to four times the standard shock dosage, often exceeding 20 ppm, depending on the pool’s cyanuric acid (CYA) level. Liquid chlorine (sodium hypochlorite) is the preferred shocking agent for saltwater pools because it rapidly increases free chlorine without introducing calcium, which can damage the salt chlorine generator (SCG) cell.
Before applying this massive dose of chlorine, turn off the SCG and remove the salt cell entirely. Exposure to extremely high chlorine concentrations and certain algaecides can permanently damage the cell’s metallic coating and internal components. Maintain the elevated shock level for at least 24 hours to ensure the complete oxidation of the algae and its protective compounds.
After the initial chlorine shock has circulated for several hours, add a specialized algaecide formulated for resistant strains. A copper-based algaecide is highly effective against mustard algae because the copper ions disrupt the algae’s cellular structure. Copper products must be used precisely according to manufacturer instructions to avoid potential staining of pool surfaces.
Filtration, Cleanup, and System Restoration
Once the chemical treatment has been applied, continuous filtration is necessary to circulate the sanitizers and collect the dead algae spores. The pump should run 24 hours a day during this cleanup process to maximize contact time between the water and the filter medium. Dead algae will often turn a cloudy grey or white and settle on the pool floor, requiring physical removal.
The settled residue must be vacuumed, preferably to waste, which bypasses the filter and prevents the spores from re-entering the pool water during backwash. If a vacuum-to-waste option is not available, the filter must be cleaned or backwashed frequently, especially when the pressure gauge indicates an increase of 8 to 10 psi above the clean operating pressure. This frequent cleaning is important for cartridge and diatomaceous earth (DE) filters, which can quickly become clogged with fine algae particles.
Brushing the pool surfaces must continue daily, even after the algae appears to be gone, to ensure no lingering spores are missed in shaded areas or crevices. The treatment is considered successful when the free chlorine level stabilizes and an overnight chlorine loss test (OCLT) confirms that the water is no longer consuming excessive chlorine. Once the free chlorine level naturally drops back down to a safe range of 3 to 5 ppm, the salt chlorine generator can be reinstalled and reactivated.
Long-Term Prevention of Recurrence
Maintaining a consistent, adequate free chlorine level is the most effective defense against mustard algae re-establishment. For saltwater pools, this often means running the SCG at a slightly higher output or for longer cycles, especially during periods of high use or hot weather. Consistent water testing to monitor pH and alkalinity helps ensure the chlorine produced by the generator is operating at its highest possible efficiency.
Regular manual brushing of all pool surfaces should become a routine practice, as this physical action removes invisible spores before they can colonize and multiply. Ensuring that the pool’s circulation system is optimized, particularly in dead spots or shaded areas, prevents the stagnant water where algae thrive. Some pool owners choose to use a maintenance dose of a quality poly-quat algaecide as a proactive measure against future outbreaks.