This unique organo-copper complex is effective against most common algae and safe for all pool surfaces. There are over 30,000 species of algae. Some algae, like blue-green algae, float in the water while others, like mustard algae and black algae, cling to the surface and thrive in cracks and crevices. There are even species that are resistant to chlorine. One application of 90-Day Algicide lasts three full months, an entire swimming season in some parts of the country. 90-Day Algicide has a slow-release property that kills active algae and prevents outbreaks. That same special formulation prevents copper from dropping out of solution, so it is guaranteed to be non-staining in a properly balanced pool. 90-Day Algicide helps chlorine work better and is safe for use with most sanitizers and salt chlorine generators.