Mud Vs. Sand

I prefer nothing. Little waterflow makes things settle out to the bottom, no sand or mud allows it to be suctioned out easily. If you're hoping for nitrate removal with a deep bed, that's what the macroalgae is for. I'm not sure what kind of refugium you're looking to set up, though, so it completely depends on what you're going for. Just thought I'd mention the option.

 

I like rubble in mine. Give the pods a good place to breed. I have macro in 1 side & the other side will have mangroves when the weather is warm enough to ship them.

 

Mangroves do not need substrate, but they will do better with it.  Mud is fine, as is any fine sand.  Once established, mangroves and a DSB can do wonders.  Mine has been running for several years, zero nitrates, zero phosphates. 

 

A good method on mangroves is to float them with styrofoam. The mangroves will throw down a bunch of roots, raise the mangroves up a little at a time & it will promote alot of root growth.

 

I like dsb's in refugiums. I would expect that most of the settling would occur in the first chamber of the sump pre-entry into the DSB area. Once established they seem to stabilize a tank very well.