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clay/laterite slurry in a plenum

I'm tempted to just keep quiet but these plenum ideas keep getting more
"imaginative" all the time. I don't like to criticize anyone's ideas but
I figured you'd want me to be honest and constructively critical.

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.

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. 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.

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
and dissolved.

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.

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.

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.

Steve                         in sunny Vancouver, BC, Canada