Going with a thin layer (1in or less) is going to be pretty much just for looks and surface area for bacteria. In my opinion if you are going to do this I would gravel vac it atleast once every few months just to get all the waste that collects in it. For some reason people online believe doing so kills all the good bacteria. It does harm so of the bacteria but it is getting all the crap out of the sand bed that collects over time. And the bacteria will just establish on the sand again. This is why I do not like having anything more then a inch, Cause you cant do anything with it without the risk of releasing hydrogen solfide.
Going with anything more then that you are going to be getting anarobic bacteria which will create hydrogen solfide, If that were to get stirred up by a rock falling, powerhead falling, ect, You wouold crash your tank. If hydrogen solfide is released into the tank it will cause a PH spike.
Going 4in or more will give you want people called a DSB (in mu eyes a ticking time bomb) and that will give you the anarobic bacteria and hydrogen solfide and will give you SOME denitrification. But the amout it will give you is really not worth the hassle. I would love to see a tank that has been setup for three years with a DSB that has not crashed or moved the tank.
DSBs and plenums all started at a local aquarium (cant remember the name of it) and they decided they wanted to give it a try. So they did and were having amazing success with it. And everyone decided to jump on board with it. But what they did not tell people is that they had a open system. Meaning they were tied into the ocean and could do 100% water changes with ocean water when ever something went wrong (which did happen many times, They just thought they would keep it to them selfs).
