Nitrogen gass is going to happen in a 1.5 inche sand bed too. What you don't want, is hydrogen sulfide to leach out into the tank...
There are many debates about this and to be honest, I believe there are only 2 ways to go, from all of the reading I have been doing from some of the top marine biologists.
1. No sand at all and 1.5 to 2 lbs of LR
6" deep with a 1" plenum built under the sand, and 1 to 1.5 lbs per gal LR
Number 1 is completely reliant upon the LR
as Filtration whereas number 2 is using the dead water in the plenum underneath the sand for de-nitrification. It is possible to run a system like this without the use of skimmers and de-nitrators and run very, very low in nitrates if any at all.
The thing to number 2 is the sand gives an appealing look to the tank as well as serves its purpose for filtration as well as the LR
. I think this is the better way of doing it and I believe many, many agree. I haven't heard any differences in doing the 6" DSB
with or without the plenum in place so, I can't help on that.
Some of the old timers that have 6" DSB
's will comment and tell you more about it...