To rule out pollen or dirt, the next question to ask the homeowner is if the ‘dirt’ returns right after brushing the pool walls. If the answer is “yes,” it is not dirt/pollen as this debris would simply drop to the bottom of the pool and would not cling to the walls. Therefore, if the pool water is not cloudy and the ‘dirt’ is clinging to the walls after brushing, the problem is likely yellow/mustard algae.
This algae is quite resistant to high chlorine levels and will grow and thrive in a well-balanced chemically treated pool. In fact, the pool’s maintenance can be by the book, but in the case of yellow/mustard algae it will still grow. It is often introduced to pools via lakes, ponds, wind, rain, and even skimmers/leaf rakes, vacuum hoses/heads used in other pools which are diagnosed with this algae.
Although yellow/mustard algae can be easily brushed off of the pool’s surface, it will return in the same place a few days later—especially in shady parts of the pool. When treating this algae it is important to use an algaecide or chlorine enhancer that specifically targets yellow/mustard algae. Sodium bromide will eradicate this algae rather quickly; however, it is only good for a one-time kill. In other words, it will not prevent this algae from returning the next time it rains. That said, for long-term protection, use a chelated and a broad spectrum copper-based algaecide to help prevent it from returning.
The importance of routine maintenance
Regular maintenance helps prevent algae growth; therefore, not only should the water be chemically maintained, the pool should also be physically cleaned (e.g. brushing, vacuuming, cleaning skimmer/pump baskets, etc.) to make it less likely for algae to bloom. It is also important to make sure the pump(s) and filter(s) are operating at least eight to 10 hours a day in the summer. Algae treatments are only effective if the pool water is balanced and is being filtered and circulated properly. Further, it is important to ensure the pool’s filters are cleaned just prior to fighting an algae problem.
When dealing with algae in an older pool with rough plaster, the pool should be resurfaced or, at the very least, any rough areas should be sanded smooth.
Terry Arko has more than 40 years of experience in the pool, spa and hot tub industry, working in service, repair, retail sales, chemical manufacturing, and product development. He is a certified pool operator (CPO) instructor through the National Swimming Pool Foundation (NSPF). He also serves as instructor for the Pool Chemistry Training Institute (PCTI) to certify residential pool techs. Arko is an active member on the Association of Pool and Spa Professionals (APSP) Recreational Water Quality Committee (RWQC). He is a member of Pool & Spa Marketing‘s Editorial Advisory Committee and currently serves as a water specialist for NC Brands, parent company of SeaKlear, Natural Chemistry and Coral Seas. He can be reached via e-mail at email@example.com.