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Re: black stuff in substrate

Thomas Barr was espousing fluorite and the fact that it has no organic
material in it and therefore won't get stinky in the water.

If you use topsoil (regular dirt, the black stuff plants grow in), then
it normally contains a certain amount of organic material. When you
submerge it in a substrate, bacteria are going to begin acting on it.
Once the majority of oxygen is gone from the soil, the kinds of bacteria
that can exist change; facultative and obligate anaerobic bacteria begin
to break down organic material using nitrates and other chemical
compounds as a source of energy. The redox potential (see my web page)
is a gross indication of the equivalent electro potential of the kinds
of oxidation and reduction reactions which are prevalent in that zone.
As oxygen and nitrates become scarce, sulphates become an energy source
for meaner and tougher breeds of bacteria (so-called street bacteria).
When you get to this level, it starts to get smelly. In fact it starts
to get smelly as soon as the bacteria start reducing nitrates to

Is this harmful to the plants? Not really. Aquatic plants are adapted to
these reducing environments and provide oxygen via their roots to
provide chemically safe buffer zones where favourable chemical reactions
can take place.

The real problem with using topsoil or soil with labile organic material
in it is not so much that it becomes anaerobic. Actually all substrates
are anaerobic since they have low oxygen levels and no air. Peat is an
organic material which has been acted on by decomposing bacteria in an
acidic environment for hundreds of years and is relatively stable.
Organic material does have its beneficial aspects and for that purpose I
recommend a small amount of peat when its used in conjunction with a
mineral soil. The peat particles provide reducing zones where iron can
be reduced to a soluble state and the peat releases humins which act as
chemical taxis for conveying iron to the roots and leaves of aquatic
plants. You don't need very much to do this job, although a surplus
doesn't seem to be much of a problem except that it colors the water
yellow and can be a little messy since it tends to float a little for
the first week or so if you stir it up. Oh, yeah, it still stinks once
its wet but its not harmful to the plants or fish. Use it with
killiefish all the time.

The problems with topsoil or soil with labile organic material are

1) they create oxygen demand. If you have a tank full of growing plants,
this is not a problem. Too few plants and the fish may not get enough
oxygen. After 3-4 months, the oxygen demand is greatly reduced as the
bacterial gangs have fought it out and used up most of the free energy

2) they can sustain a very very healthy bacterial and diatom population.
This creates a substantial layer of slime on plants initially. It can be
a big problem for plants since it cuts down on light, respiration and
the ability to get nutrients from the water. If you have a good
population of snails (Ramshorn snails are probably the best choice, they
reproduce rapidly but are not as plant hungry as other "pond" snails),
they will eat this layer of bacteria and diatoms. They will eat it so
fast that three or four large ones can literally clean a badly affected
tank in two days.

Topsoil can be comprised of many things. It can be mainly sand with a
large organic mulch component such as I have, or it can be a mixture of
organic material, sand, silt and clay. As far as a good soil for mixing
with the substrate, I think you want clay or silt with a little clay in
it. That clay is the stuff that has the iron and other trace minerals in
it. The best stuff I've gotten so far was McKenzie river muck which
seems to be a very fine silt and clay mixture. I did have some slime
with it initially but the addition of a few Ramshorns took care of that
in a couple days. The growth rates in that tank have been very good and
I've not added any additional fertilization to that substrate.

If you want to find a good soil to use in a substrate, I recommend a
clay mixed with a couple handfuls of peat and mixed with sand or fine
gravel. Just like laterite, you don't need a lot since the clay
particles will form a coating over the sand particles and there is a
huge surface area where minerals can be solubalized.

You can find clay all over the place; any muddy spot will do. Avoid mud
that is in an arid region as it may be highly alkaline. Instead look for
a clay deposit on a hill.

Its probably wise to use either soil with a little organic material or
peat, if you suspect that it might have toxic amounts of minerals like
lead, copper salts etc which can be present in some soil especially if
not adequately leached by natural rainfalls. The peat and organic
material absorb free metal ions and renders those soils non-toxic.

Or, if you're the fretful sort; go buy some Fluorite or laterite or one
of the pottery clays which have been successfully used.
Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!