Ping would be a great person to talk to about this. I have zero sand in my fuge as it was leaking but I put a 1 inch sand bed in mine as my display has a 3-4 sand bed. From what I remember is fuges can be set up a couple ways. Cryptic fuge- has a bunch of sponges that filter the water. DSB- which helps to convert nitrates, etc. Macro- which usually has a shallow sand bed but have seen some with dsb. These use macro to convert nitrates. Im sure there are others but these are the main ones I can think of. Again I would talk to PING as he has tons of refugiums.
Kevin- 375 Gallon Reef
Reefing made easy...