Other forms: algaecides
Algaecide is a substance that kills algae. It is usually used to keep pools, fish tanks, and similar things clean.
The -cide part of the word is from the Latin cida, meaning "killer" or "killing." So algaecide is a substance used to kill off unwanted algae. Algaecide is usually used for pools or damp areas where algae grows without its natural predators: fish.