Slimey Black stuff in Rocks.

[Ron_Vas]

Not my pic but have the exact same s*** growing on my rocks. Anyone know or have an idea of what it can be or if it will harm my fish? No corals in there. Haven't added any new live rock in years.

 

[jadakiss]

It looks like some sort of sponge, doesnt look like cyno. How old is your tank

 

[Ron_Vas]

4 years old. Parameters are good. Just realized it today after transferring my tank.

 

[35ppt]

Sponge, just scrape it off if you don't like the look of it. You could also look at reducing silicate if needed and feeding of particulate foods