I have a 210G BB SPS dominated tank with a 75G RDSB w/ 6" of oolithic sand (~345#). Total water volume is ~ 300G. I have 4 medium size tangs (4" or so) that I keep well fed. The setup has been up and running since 10/2007, however, all livestock except for the LR and sand came from a prior setup (110G total water volume w/ ~100# of LR) that did not employ a DSB of any kind.
The RDSB has been in place since the beginning. Flow through the RDSB is ~400GPH. I also have a small power head (Aquaclear 201) in the RDSB for some additional flow. I stir the top inch or so monthly. The RDSB is not lighted.
In my previous setup (~110G total water volume w/ ~100# of LR), I struggled with nitrate levels ranging from 5-10ppm. I performed 25% water changes monthly. That system had been running for ~2 years and the nitrate levels were consistent throughout that period.
In my existing setup, (~300G total water volume w/ 150# of LR and ~345# of live sand) nitrate levels are undetectable. I perform ~10% water change every 3 weeks or so.
General Comments:
I'm glad I employed the RDSB. I believe it's helping control nitrates as well as aiding in maintaining PH and Alkalinity, not to mention the added water volume.
It also makes doing water changes easier, as I can take it off line and perform the water change via the RDSB.
In my opinion, I also feel I'm able to run less carbon than I had in past systems while maintaining similar or better water clarity. I have no quantitative data to judge water quality to support this, I just feel I'm able to maintain better water clarity w/ less carbon.
For comparison purposes, my current system has much better flow and much better skimming, which no doubt also contributes to the improved water quality.
Also, the added water volume in my current setup makes the system more stable and overall maintenance much easier as well, however, if I had to do it all over again, I would still employ the RDSB.