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Re: Aquatic Plants Digest V3 #323

In Aquatic Plants Digest V3 #323, Tom Wood asked:

>I'm still experimenting with this PMDD plenum tank and it's starting to
>respond.  When I set it up I mixed Dupla laterite in the lower part of the
>gravel, just above the plenum.  After reading the archive discussions on CECs
>of various materials, I'm wondering if I should attempt to add some such
>material to the plenum space.  The reasoning being that water movement 
>the plenum would carry water borne nutrients which would attach to laterite
>(for example) in the plenum and help hold ithe nutrients away from algae in
>the water column and next to any roots that make their way to the plenum
>space. ...<snip>

I would go carefully with this approach if I tried it (in a test tank 
only, not a show tank)- it changes the nature of the plenum drastically.

On second thought, I'll qualify that to "it changes the _volume_ of the 
plenum drastically".

On third thought, I'll leave thoughts 1 and 2 in because they may both be 

My understanding is that the plenum processes are driven by diffusion - 
there is a significant water volume below the substrate that receives 
water only through the substrate. The makeup of the water coming through 
the substrate has been changed from that in the water column above due to 
processes occurring in the substrate (including nutrient uptake by roots 
and at cation bonding sites, as well as some biological filtration 
processes) so an analysis of the plenum water will be different to that 
of water above the substrate. Since both water spaces are connected (it's 
only one body of water really), the whole body of water "attempts" to 
reach a uniform state by water from the tank space flowing to the plenum 
and vice versa, in order to mix the contents. The slowness of the flow 
rate through the substrate, and the processes occurring in the substrate 
during the flow, prevent mixing to uniformity. In terms of water flow, we 
could think of it almost like a free pump.

I think that the volume of the plenum is one of the factors driving the 
flow process. In the reef tank material I've read, the minimum height of 
the plenum is critical and, seeing there's nothing in it, that 
effectively sets a minimum volume. Place something in it and you reduce 
the water volume in the plenum. You could compensate by increasing the 
height if that is the only change introduced by adding some material.

I suspect that the oxygen levels in the plenum are lower than in the 
substrate which are definitely lower than in the water above the 
substrate. Biological processes _will_ occur in whatever you put inside 
the plenum, and they will be different processes to those occurring in 
the substrate because of the lower oxygen levels (different bacteria will 
be supported by different conditions). I don't know whether those 
processes will be beneficial or harmful to your tank. I only know one way 
to find out, and it may or may not be pleasant.

David Aiken (AGA member)