Plenums are a proposed method for degrading nitrate to N2. They increase the filtration capacity by making the substrate area more efficient at filtering biologically. Plenums are normally setup over the entire base of the tank using a void and a thick layer of gravel.
Last year
I added a plenum in a box (PIB) to one of my 6′ tanks.
Essentially, this was a 15 x 20 x 20 plastic container
in which I placed some PVC pipe spacers (height 4cm) and
a layer of egg crate. Over the egg crate I placed a
piece of fibreglass flyscreen and filled the container
with pea sized gravel (the depth of which was ~15cm)
The container I used was semi-transparent, and I didn't notice
any bubbles forming (a sign that nitrate may be being reduced to
atmospheric N2). Once installed for a few months I
began to notice brown algae growing on the white PVC pipes which
held the egg crate 4cm off the bottom of the PIB. This
Presumably indicates that there is enough oxygen for brown
algae to grow. If I had to guess I’d say O2 levels
are still to high in my PIB. This has implications for those
people setting up freshwater plenums in new tanks with regard to
the grain size and depth of the gravel. I’m working on a new
version presently with fine gravel and a larger void. I’ll keep
you posted.
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