Dark patches of algae?? Maybe something else.

[zheka757]

In 5 years of owning tank, don't recall seing this type of growth on rocks/tank walls. It's growing over coraline algae also. Any idea what it is? Ok to have?

 

[Dan_P]

In 5 years of owning tank, don't recall seing this type of growth on rocks/tank walls. It's growing over coraline algae also. Any idea what it is? Ok to have?

Could be cyanobacteria if it is soft and easy to scrape off. It can form a thick film on surfaces.