Spot algae is a type of algae that appears as small, dark green or black spots on aquarium surfaces, such as glass, plants, and decorations. It is a common issue in aquariums and is caused by excess nutrients, such as nitrates and phosphates, in the water. To control spot algae, it is important to address the underlying cause of the excess nutrients. This can be done through regular water changes, reducing feeding, and ensuring proper filtration. Manual removal of the algae with a scraper or brush can also be effective. Additionally, introducing algae-eating fish or invertebrates, such as snails or shrimp, can help keep spot algae under control.