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Re: Plenums, slurries, laterite and such.
Steve wrote several good comments regarding the use of plenums in freshwater
aquariums, but I'd still like to pursue it, even if it is only "imaginative".
1) I don't think it will be practical since clay slurry is not going to
be easy to filter out of the circulation water.
This was just a way to get it into a tank already set up, there is laterite in
the gravel and it doesn't wash out, so I doubt it would get out of the plenum.
I'd expect the laterite slurry to settle into a thin layer at the very bottom
of the tank.
2) why bother with plenums when they do nothing to improve the
availability of phosphates in the substrate? In fact, they will tend to
circulate interstitial phosphates and other nutrients back into the
aquarium water, exactly where you DON'T want them.
This gets close to answering my original question, which was, does laterite
and/or vermiculite have properties that cause them to attach to and hold on to
such nutrients? If so, and such materials are present in the plenum, would
they effectively sequester such nutrients in the plenum?
2) continued: Iron, phosphorus and other low solubility minerals are the ones
which are advantageous when supplied in the substrate. Soluble nutrients such
as nitrates, Ca, K, Mg and S do not need to be absorbed into the substrate by
a plenum circulation system in order for aquatic plants to use them.
I thought that the substrate heating cables were intended to do exactly this?
I'm wanting the plenum to serve as a passive replacement for the heating
cables since PMDD is a totally waterborne fertilizer method.
3) plenums and other open spaces beneath the substrate only collect iron
oxides and other insoluble precipitates where they are not available for
plant roots. It is better to have a continuous substrate so that
precipitates settle into the anaerobic zones where they can be reduced
It's my understanding that plenums -are- anaerobic.
4) why don't you START with a higher concentration of nutrients in your
substrate rather than trying to absorb them from the water? If you
circulate water through the substrate, it is more likely to release the
nutrients INTO the water, not out of it IMHO. The easiest way to
increase the fertility of your substrate is to add a small amount of
solid fertilizer inside a clay ball. I suggest about 10 granules of
fertilizer in a 1/2 tsp. ball of clay used 1 clay ball per large plant
to be adequate. Don't over do it because the nutrients do get out for
the other plants too.
Steve - I've read the archives and your website, I know you advocate the
rich substrate method. I'm experimenting here and don't understand the
chemistry as well as you, but I like the controlled dosage method that PMDD
offers. If I mess it up, a couple of water changes gets me back to square
5) It is not difficult for aquatic plants to absorb the majority of the
excess nutrients released by fish feeding when you have a well planted
aquarium with adequate lighting and other supplemental nutrients. You
don't need to resort to any plenum device (ineffective as they are
likely to be). I think the best way to deal with excess nutrients in
your aquarium water is to a) avoid adding too much iron fertilizer b)
keep the plant to fish ratio high c) practice frequent water changes if
you have a high fish load d) ensure that you have enough light and other
nutrients so the plants can grow rapidly.
After ordering and reading the ECO catalogue on hydroponics, it struck me that
they are doing the same thing that PMDD does, we just have the additional
problem that a fully submerged condition presents. I'm looking at the gravel
substrate as serving the same purpose the rockwool that hydroponic systems
use, and the plenum as the place where nutrients are collected and fed to the
roots. The denitrification benefits that plenums serve in a reef are probably
counterproductive here, unless pure nitrogen is easier for plants to
assimilate. Is it?
5) Certain botanists have demonstrated that a water transpiration system
exists even in FULLY SUBMERGED aquatic plants. This water circulation
stream draws water down into the substrate, into plant roots and up
through the plant stems to the actively growing regions of the plant
where they can be absorbed. This phenomenon was documented in a recent
article in TAG 10:6 by Ole Pedersen of the University of Copenhagen. It
was originally published in Aquarium Heute No 3, 1996.
Okay.....Not sure what this means. Anyway, the basic question remains, would
laterite and/or vermiculite in a plenum act to hold nutrients?