what exactly are you trying to do?
never really heard of a machine to harbor denitrifying bacteria (which converts nitrate to nitrogen gas).. Do you mean nitrifying bacteria, or what?
EDIT- Ah, I see. Here's a link-
Coil Denitrifier
so they're usually used in marine tanks? In freshwater, I'd rather just have plants to control nitrates. In marine, there's deep-sand bed and protein skimmers.
Unplanted freshwater? I'd stick to water changes.
If by some crazy fluke I decided to make one, I'd go with something like estes reef sand or pool filter sand.