mud filter????

[mattistat]

what is that?

 

[saltwater4life]

Filtration - Mud Filters : Aquariumpros.com
it's a type of filter just like a bio wheel filter
(i'm guessing i don't know for sure but i know thats what those are)

 

[S.Reef]

It generally is a filter specifically meant for reef aquariums. it utilizes an area that contains a "mud" that has acts as a bio filter and at the same time leaches out beneficial minerals into the water for the fish and corals.