Black sponge?

[icejam]

Is this black sponge?

About 1¢ coin in diameter must have been there a while but only noticed it today.
Is inside of chimney like formation with less flow than in the rest of tank.

Should I let it be or chesel it off?

Thanks.

 

[Sabellafella]

have some black sponge hanging around my tank for a long time now. Doesn't bother anybody. Actually really like the black one over the other colors I've found.

Is this black sponge?

About 1¢ coin in diameter must have been there a while but only noticed it today.
Is inside of chimney like formation with less flow than in the rest of tank.

Should I let it be or chesel it off?

Thanks.