What is the black specks on aquarium rocks?

The black specks on aquarium rocks are often a type of algae, specifically black beard algae or cyanobacteria, which can thrive in aquariums due to excess nutrients, low light, or poor water circulation. They can also be remnants of decaying organic matter or mineral deposits. Regular maintenance, such as cleaning and optimizing water conditions, can help prevent their buildup. If the specks persist, consider adjusting your tank's feeding and filtration practices.