An "as-needed" chemical. If algae does occur, Algaecides are used to help kill algae. The majority of algaecides are liquid, but some types do come in granular form. Once you determine the type of algae (green algae, mustard algae, or black algae), you can purchase the appropriate algaecide and begin the proper treatment. Then read the instructions on the label to determine the amount to add, how it should be added (most manufacturers of algaecides recommend pouring it straight from the bottle), and other precautions. Note: In conjunction with the algaecide, you will also need to use a chlorine-based shock and engage in a labor-intensive and time-consuming maintenance schedule to eliminate the algae.