Anaerobic vs. Anoxic terminology for substrate
The following is in part a posting which I made recently on
the rec.aquaria.freshwater.plants news group.
There seems to be a lot of confusion and misconceptions about
"anaerobic" substrates and the danger of H2S affecting either
fish or plants.
1) all substrates are anaerobic. It simply means without air. By
generalization of the term it also is meant to imply anoxic which
means without oxygen and to a great extent all substrates are also
anoxic. This is natural and not a problem. Bacteria acting upon
organic substances (either soil components or fish poop or detritus)
consume the oxygen. Oxygen diffuses into the top layers of the
substrate for about half an inch (dependent upon a lot of factors).
Deeper in the substrate other bacteria take over and using fermentive
(sp?) processes use nitrates, sulphates and various other compounds
to provide energy to support their cellular activities. One of the
products of these bacteria is our old friend ammonia. So long as
the concentration of ammonia doesn't get high enough to bother our
fishy inhabitants, the ammonia is beneficial and is used more easily
by plants as a source of nitrogen.
We should all stop using the term anaerobic to describe substrates.
It is entirely misleading and does not accurately describe the bad
things you can have happen in your substrate under certain conditions.
Substrates composed primarily of minerals such as clay, sand, laterite,
gravel or rocks, no matter how densely packed will never have a problem
with excess H2S or ammonia.
2) it's my opinion that in an organic substrate (and only in an organic
substrate) the real problem is the excess production of ammonia and
its effects upon the fish. The plants seem to thrive in such an
environment. (most plants that is) It is important to have a balance
between the amount of organic material and the amount of plants growing
in the tank and the supply of other metabolic requirements (light, CO2,
micro-nutrients) to allow the plants to consume the ammonia.
I would like to see more experimental work done to establish just how
much organic material of various types we can use in a substrate. A
layer of about an inch with an organic content of 5% seems quite safe
whereas a similar layer with about 50% organic content produces a
substantial amount of ammonia. Peat for example is highly acidic and
one could use quite a lot of it without danger of too rapid
decomposition. Add lime to peat and it might decompose much more
The substrate which I referred to has a one inch layer of about
50/50 vermiculite and earthworm castings and was enriched with
slow-release fertilizer prills which have a polymer coating.
Earthworm castings are a very fertile source of organic materials
which are fairly well composted. As such they are about as fertile
as you can get without using less completely composted vegetable
material which can have an extremely high oxygen demand and produce
a great many unwanted and potentially toxic forms of bacteria.
In choosing these materials I wanted to test the limits a little.
It this point the ammonia level in the tank seems to be hovering
at around .25ppm which is higher than I'd like it to be for fish.
The current fish population in this 50 gal tank is 7 otocinclus
and 5 mature head & tail light tetras. Since the otocinclus
have adjusted to the tank, there have been no fish mortalities
except for a female platy which I cannot locate and which I
suspect may have suicided by leaping over the edge of the
I have introduced faster growing Ceratophylum and Salvinia to
help soak up the ammonia. Most of the plants in this tank are
a little more slow growing because it's intended to be a show
tank. About 1/3 of the area is devoted to various Crypts.
I should also mention that there is no filtration of any kind on
this tank. All of my tanks currently rely upon plants to consume
ammonia. Since I expect the rate of ammonia production may
increase before it decreases, I will probably add a biological
filter to the tank.
The first algae which colonized this tank were soft green attached
algaes. The otos have this well under control. The next algae
was suspended green water algae which I removed using a diatom
filter. Blue-green algaes were the next and I have been
battling with these by removing and cleaning affected plants
and vacuuming affected gravel well. It seems to be declining
but the tank must be carefully watched for Blue-Greens and
colonies removed each day.
As long as the tank remains viable I will keep making observations
and reporting them from time to time. I expect that there may be
several transitions in the substrate as anaerobic (anoxic)
bacterial colonies are established and replaced by others types
and as the nutrients are gradually bound into the plant mass or
removed by water changes. It would be extremely interesting if
I could make more analytical measurements of the substrate
redox potential plotted vs. depth and time. I would also like
to analyze the various gases which are escaping from the substrate.
At the current time, there are regular escapes of bubbles and if
I disturb the substrate, I can release a fairly good sized stream
of bubbles. I suspect that the majority of this gas is methane.
I may collect some to see if its combustible. If I got enough
I could make a pretty respectable explosion! ;-)
Despite all this bubbling, the fish and plants don't appear to be
suffering any ill effects. Appetite and activity seem good. I am
maintaining a pH of 7 or lower via CO2 injection. It's fairly
critical to maintain a low pH to prevent toxic effects of the
ammonia which would occur at a higher pH. At this point it looks
like the rate of CO2 consumption by the plants has increased.
I'll check the bubble rates and CO2 concentration to verify.
Steve Pushak - spush at hcsd_hac.com - Vancouver, BC, Canada
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