An optional chemical. Algistats are used to help prevent algae. Some pool owners will use an algistat in conjunction with their other mandatory chemicals in order to help prevent an algae outbreak. Algistats are typically packaged and sold as "Preventative Algaecide," or "Maintenance Algaecide." Algistats are liquid. Read the instructions on the label to determine the amount to add, how it should be added (most manufacturers of algistats recommend pouring it straight from the bottle), and other precautions.