Sponges? Or something bad?

[kyle.nagel13]

Tanks been at up for 3 months, nothing but rock in it. Just noticed these tonight and I can't seem to find an identical picture. Closest I can find says they are sponges, but I'd appreciate a second opinion.

 

[Timfish]

My vote is sponges

 

[Newguy76]

+1 for sponge

 

[kyle.nagel13]

Thank god, I haven't been running the lights and just noticed them for the first time last night and pretty much freaked out.

 

[whitrose02]

Yep lightning sponge. They usually grow in sumps. Totally harmless.

 

[dgbandog]

is there any sponge growth that you should be worried about?