I want to pick up on a thread that was discussed here
a little while ago - under-gravel heaters.
Kasper Horst of Dupla fame, wrote in his book
"Pflanzen im Aquarium" that _on average_ aquatic
plants can absorb nutrients from any surface [i.e.
roots, stem, leaves], that they can transport
nutrients, and metabolic end [and by!-] products in
ANY direction within their structure, and they
excrete compounds into their environment. Diana
Walstad, in her excellent paper on allelopathy 
published in TAG, elaborates on this and gives us
actual examples and data.
Let me apply this to the substrate of a new tank. It
will go through a number of stages:
1) New plants start growing their roots in the
substrate. Because there are no allelochemicals
present, the initial spreading of the root is not
limited by them, and bacteria start to populate the
2) After some time, the allelochemicals excreted by
the roots become concentrated enough to kill
bacteria, and rootlets of some plants. Some of the
released compounds are not easily broken down and
3) After a long period of time (years(?)), the
substrate becomes "old", saturated with growth
inhibitors that slow root growth, and consequently
reduce O2 release from the roots. The accumulated
organic matter is broken down, using up yet more O2
The result is poor plant growth, a phenomena hinted
at by Horst.
4) In the final stage, the substrate become anaerobic
and needs to be replaced. 

Enter under gravel heaters:
Instead of looking at the UGH (or any other form of
water-through-gravel circulation) only from one
perspective (providing roots with nutrients) we could
look at the other side - removing allelochemicals
from the substrate.
I believe that UGH will flush the nasties out of the
gravel and into the water column where they can be
dealt with during water changes.
Further, I speculate that the increase in temperature
of the gravel breaks down the compounds faster. To
me, the Dupla approach (high temp) is more in line
with this thought than the low temperature Dennerle

Therefore, for long-term substrate management,
the UGH seems to be the way to go.

(...a Pandora's box...hehehe...)
Subject: Welcome to the World of Anubia
Anubia aseems to be the female form of Anubius
who was a god in the eqyption mythology.