Hi Hodgie.
The plenum,
Grain sizes recomended 1.75mm to 4mm smaller grain sizes can block up meshes and bind.
Besides being
an area to contain a slight bit more oxygen than the above sandbed, is also
a normal and needed reservoir for certain constituents such as nitrate.
Nitrate and other constituents are temporarily store there and used by the
bacteria above in their metabolism processes and/or as foodstuffs. Draining
the plenum only upsets the processes and harms the balance/equilibrium in
the area. People who had drained their plenum and found nitrate above that
in the bulk water panicked and wrote unflattering articles about the plenum
system thinking they had discovered a fault. Little did they realize what
they saw was a normal and helpful attribute of the system.
My understanding on the plenum using aragonite sand
The semi-stagnate cavity mimics the reaction that take place in deeper sediment beds of the ocean.
different bacteria populate different parts of the structure according to the amount of oxygen that gets there.
These bacteria reduce the amount of three waste products that are found in the water of the entire tank.:
ammonia nitrites nitrates.
these bacteria also slightly acidify the water in the bed to slowly dissolve the sand.
This is called dissolution.
If the PH in the aquarium itself is low it can dissolve more of the sand,which releases buffers.
These buffers work to bring the water back into the normal range. (sounds good?)
As the sand dissolves it also helps regulate several element levels like calcium,carbonate and strontium .
Microbially, crushed coral would provide the same benefits as would
aragonite. The only difference is that aragonite does help supplement the
carbonate buffering system somewhat. Where it is too expensive, crushed
coral will suffice and alkalinity can easily be controlled via a calcium
reactor
As far as I see a plenum it creates a nitrate reduction system that's it and in certain circumstances it may be more helpful than a deep sand bed for nitrate reduction I.E. a shallow reef crest set up with very high water flow may keep kicking up very fine sand not good for SPS corals.
An infauna live deep sand bed will also reduce nitrates as well as a plenum but a deep sand bed due to fine grain size will support a diversity infauna that will give the added benefit over the plenum.
They will keep the bed turned over and remove detritus and will give a diversity of natural live plankton larvae to the system that a plenum will not, which is a major advantage.
Also a plenum needs the top inch regularly disturbed more maintenance involved this is.
Bob Goemans recommends the grain sizes for a plenum be between 1.6mm to 4mm
Rob Toonen info on.
Southdown Tropical Play Sand
Size breakdown of the sand:
>500 um (= 0.5 - 2.69 mm): 14% (largest particle 2.69 mm)
>300 um (= 0.3 - 0.5 mm): 32%
>250 um (= 0.25 - 0.3 mm): 16%
>180 um (= 0.18 - 0.25 mm): 20%
>125 um (= 0.125 - 0.18 mm): 13%
>100 um (= 0.1 - 0.125 mm): 3%
>50 um (= 0.05 - 0.1 mm): <1%
<50 um (= 0.001 - 0.05mm): <1%
Ideal for a DSB=(Deep Sand Bed) but not for a plenum.
A little light reading below.
Sandbed References
HTH
Martyn
[This message has been edited by Martyn (edited 10-16-2000).]