GROWS ON A STONE

[Genvid]

Can anyone identify what this is on my rock. I think it's a sponge, but I don't know if it's dangerous for a tank or not. It's dark blue.

 

[MoshJosh]

Yeah that's a sponge and it's "probably" not dangerous. There is a variety of coral killing black sponge but dont think it looks like and I think it's pretty rare in the aquarium