Black Sponge???

[Johny Cash]

Hello
I realized these guys are spreading fairly fast, what is this??

black in color encrusting

 

[Neoalchemist]

You called it right. Black Sponge grows too well in an aquarium. I dont know of any problems associated with them other than them taking up real estate. They grow fast and will not confine themselves to shady areas, but usually wont' grow into very bright areas. I dont know of anythkng that eats it or any other ways to control it. But since its worst offence is that its ugly i leave it alone.