RDSB are often kept entirely for nitrate removal, with no introduction of critters or macroalgae. Light is excluded from them and a good flow of water is flowed over them. A fast enough flow is kept running over them that no particulates settle on the sand. A two inch level of water is typically maintained over the sand. It is very commonly used by commercial coral growers for total nitrate removal. Calfo has introduced a lot of the commercial practices to the home reef tank keeper, through his books, public speaking and through being a contributor (moderator) on other larger reef sites. They in general work better than the standard practices of the home reef keeper, but aesthetics typically are not as important with commercial growers as they are with home reefers who have tanks in their living rooms and such. I run an in tank DSB in my home display tank, but all my growout and frag tanks have no sand or rock in them. Every thing supporting them is remote. Remote deep sand beds, remote refugium/sumps, remote skimmers. Very few pumps, but a lot of air used. I could not function efficiently without remote deep sand beds in my commercial tanks. And I am a small scale grower. I gurantee large scale commercial coral growers and fish breeders could not run there tanks the way a home reefer runs their tanks. Way too inefficient and therefore too prohibitively expensive for commercial use.