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re: Sulfide gas around driftwood

I concur that the driftwood is causing an area of the substrate to have
a low redox zone; the sulferous smells are evidence of this.
Incidentally these smells are probably micro traces of methyl or ethyl
mercaptan, NOT free hydrogen sulfide. As Paul suggests, thick rooted
plants in this region will prevent this. 

I use a large piece of Mopani wood in my aquarium; in front of this I
plant Cryptocorynes and an Amazon Sword. Behind it are several Bacopa
stems. I wouldn't be surprised if the area underneath the wood goes to a
redox low enough to produce sulfides but I also wouldn't be surprised if
it doesn't. 

The high oxygen levels in the aquarium water prevent a localized low
redox zone from ever being a problem to the fish since free sulfide
cannot (effectively) exist in oxygenated water. Reactive sulfides (i.e.
H2S) are oxidized by nitrates, nitrites or oxygen as they diffuse from
low redox regions of the substrate. For the most part, plants are not
bothered in the least. A large Sword plant sends out roots for quite a
distance and can raise redox potential in some substrates above 300 mV
to where Fe reduction will not occur. Sulfide reduction occurs at 0 to
-150 mV. Oxygen is being consumed at redox 500 to 350 mV. Below 350 mV
oxygen is (effectively) absent (anaerobic). I think our aquarium water
with plenty of free oxygen will have a redox of 500 mV or a little
higher if I interpret this data correctly. 

In my opinion, hydrogen sulfide and low redox (often inappropriately
mistermed anaerobic) are the boogie man of aquarium lore. The aquatic
botanists who study substrate conditions in relation to plant growth (eg
Barko and Smart) cannot tell us definitively what causes diminished
growth in correlation to increased organic content. I don't believe it
is redox related or related to the loss of nutrients. There are numerous
byproducts of decomposition (alcohols, phenolics, esters, aldehydes,
ketones and so forth) which are relatively stable and mildly toxic and
these are the primary cause which Barko & Smart suggested in their
studies. An accumulation of feces after a year or two especially with an
UGF might cause such a condition. Just a theory...   :-)

Some of you should think carefully about what I said in the preceding
paragraph to see if it supports the assertion that laterite and similar
stable materials might be more optimal for very long term aquarium
substrates. I'm not aware of many studies which extended for multi-year
time periods. Some knowledge is not easily gained.

I have some references if anyone wants to read further on the subject.

Stephen Pushak
Aquatic Gardeners Association

"In the intellectual order, the virtue of humility is nothing more nor
less than the power of attention." -  Simone Weil
"Do not use a hatchet to remove a fly from your friend's forehead." -
Chinese proverb.