Their are a bunch of things you can put in there nothing is really right or wrong per say. To list a few refugium mud, Deep sand bed, macro algae (for example cheato).....before I had a macro tank set up on my 400g system I personally ran a DSB and cheato. Do whatever your heart desires.
I think the biggest key point whats is enough. To many folks expect insane results from a refugium a 15th the size of the DT. Think at least 50% is needed.
BUT if your talking about an empty sump space.... How is that a benefit exactly?
Least use that extra space for chaetomorpha(which apparently I've been mispronouncing forever!)
and mangroves rock....thank you very much.
I disagree with Jake...that its all simple you have THis problem it's fixed with This. End of story. Reef tanks are about balance of a minature eco system.......... its lots of different parts.
Cool thanks guys...mud and sandbed is out because om using my return chamber as refugium.its devided with live rock so the algea dosnt go in the pump. I was just curious if i could ad more to benefit from.