"Mud" is actually being used in this hobby to refer to a combination of silt, sand, and pebbles. It is most often used as a DSB in conjunction with macroalgae in a sump. I would check out this thread for more information and links to info at the "Miracle Mud" website.
"Okay, time to learn about *mud*" -- Jan. 17, 2002
Cheers,
kris