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Re: hardness and plenums

Jay asked:

> Why doesn't the aquarium hobby industry change to the "ppm" scale
> in refering to water hardness?  Maybe it would help to avoid some
> of the confusion.

I can hardly speak for the hobby industry, but I can think of reasons not
to use ppm.  Foremost in my mind is that the "degree" scales provide
results in small whole numbers.  That's nice.  Its easy to understand.

From a slightly more technical standpoint, the degree scales represent the
built-in precision of the test kits.  I think getting results in small
whole numbers discourages people from speculating about fractional parts
in between.  Results in larger numbers lets people think that maybe there
is something between 18 ppm and 35 ppm (1 degree and 2 degrees) that they
can read from their test kits, when in fact the precision of the kit isn't
good enough to allow that.


I have a plenum in all of my planted tanks.  These are left over from UGFs
that are not otherwise in use, so I doubt that they are similar to the
plenum conceived of in this discussion, but the discussion brought up two
points that I wanted to comment on:

Submersed hydroponics...

If you provide all of the nutrients through PMDD or other sources of
dissolved nutrients (as they do in hydroponics) then you will feed algae
as readily as you feed your plants.  I think submersed growth of
macrophytes w/o severe algae problems will require at least one of the
essential nutrients to be sequestered in the substrate.

Anaerobic conditions in a substrate...

Steve brought up two points that seem to be at odds with one another.  On
one hand he said that anaerobic conditions in the substrate were necessary
to keep typically insoluble nutrients like iron and phosphorus in a
biologically available state.  On the other hand he said that rooted
plants circulate water in and aerate the substrate.  If the latter were
completely true, then the former would not happen.

A point that is often missed when we discuss conditions in the substrate
is that the substrate doesn't need to be entirely, homogeneously anoxic to
provide an environment for anaerobic reactions.  Conversely, it doesn't
need to be completely and homogeneously aerated to provide an environment
for aerobic reactions.  Each pore space between grains in a substrate is
potentially a different chemical microenvironment often as small as a few
microns across.  Local conditions in the pore space are determined by the
availability of organic compounds, oxygen and other nutrients in the
immediate vicinity of the pore.

If we provide a lot of circulation in a substrate, then the circulation
tends to homogenize everything.  The tendency to homogeneously aerated
conditions is promoted if the substrate consists of large, regular grains
of uniform size - like commercial aquarium gravel.  There's little
opportunity in that kind of substrate to develop closed-off pores with
chemically unique conditions.

At the other extreme, if there's little or no circulation, then the
conditions will tend to be homogeneously anoxic, as the supply of oxygen
into the substrate will rarely be sufficient to provide for the oxygen
demand by bacteria feeding on organics compounds in the substrate.  Here's
where we get smelly and gross conditions.

In between these two extremes the substrate consists of aerated and
anoxic zones that support an intimate mixture of anaerobic and aerobic
environments.  Under these conditions the substrate should be able to
provide a regular supply of nutrients and support a healthy and diverse
microbe population that provides for the conversion of nutrients from
biochemical to (plant-available) mineral forms.

So moderation appears to be important.  Do the plants alone provide enough
circulation in the substrate to create those moderate conditions?  The
reports on this list seem to be mixed.  You could probably say that plants
alone don't provide enough circulation to create healthy conditions
in every substrate.  But in some conditions they probably provide plenty
of circulation.  Certainly, the grain size and the distribution of grains
sizes in the substrate (with a mixture of sizes being optimal) and the
amount and reactivity of organics in the substrate (organics in fish
waste are relatively reactive) are important factors.

I wonder too, if the amount of circulation provided by the plants will
depend to a large degree on how much of their nutrient supply is derived
from the substrate (thus requiring the plants to circulate water to
transport the nutrients) and how much comes from foliar feeding.  If my
guess is right, when you provide most of the nutrients in the substrate
this will promote root feeding by the plants and you will have very good
substrate conditions.  Conversely, if you provide most of the plant
nutrients in the water, then you may be promoting homogeneously anaerobic,
nitrogen-depleted conditions in the substrate.

Roger Miller

In Albuquerque, where the air is so painfully clear that objects 20 miles
distant appear to be but a short walk away.