tcwayne
OLDGUY-OG
<a href=showthread.php?s=&postid=13204649#post13204649 target=_blank>Originally posted</a> by The Hawk'ster
in a nut shell
A plenum is little more than a vacant space in the substrate, harboring anaerobic bacteria, which converts nitrates into nitrogen. In order for the anaerobic bacteria to function, the plenum must contain carbon dioxide and be devoid of oxygen.
So technically you can say it is an undergravel filter with out the up tubes. You also need to have the right layers and thickness of substarte for it to work properly.
In case I missed something, how deep of a bed do you go above that with sand?