Brown sponge filter

[Live Rock]

Hi I put a sponge filter in my tank to re-inoculate it with bacteria, but there is some sort of brown stuff discoloring the sponge. any ideas?

 

[little_fish]

could be diatoms, algae, or growth from who knows what else

 

[Smitty]

+1 lil fish...it's probably diatoms.