[APD] RE: Plant tank substrate going anaerobic

["Steve Pushak" <teban at powersonic_bc.ca>]

Berne Kairunas asked:
> when you uproot  plants [you get] large gas bubbles
> These smell really bad
> I'm wondering [about] replacing the substrate, or
> revive this with good solid plant growth and no ill
> effect to the fish.

I had a tank with a very deep substrate that included some garden soil that
I bought from a retail outlet that produced bubbles if disturbed. I also had
bubbles appear from another substrate made using a mixture of earthworm
castings & soil. Both supported heavy plant growth & large fish populations.
I would not disturb it unnecessarily. After many months, bubbles stopped
coming up.

The gas which you see is probably a mixture of nitrogen & methane from
decomposition. The nasty rotten-egg smell is the result of mercaptans, "any
of various compounds with the general formula of RSH (thiols) that are
analogous to the alcohols & phenols but contain sulphur in place of oxygen
and often have disagreeable odors" (Webster's '73). Mercaptan is used in
natural gas (odorless) so that you know when you have a gas leak. The
concentration can be very tiny and you can still smell it. In sufficient
quantity it can be toxic. H2S is extremely unlikely; its quite reactive &
not a stable decomposition product.

The good news is that your substrate is only 3" deep (half gravel) and many
other people have probably used a similar design before. I suspect that you
have a very good chance of not suffering any fish mortalities.

On my website I offer directions <http://home.infinet.net/teban/how-to.html>
for using soil in aquariums & precautions
<http://home.infinet.net/teban/soil.precautions.html> for the paranoid. Note
that I don't recommend for general use potting soil alone nor rich materials
such as compost or earthworm castings. Of course, you can use these if you
know what you're doing. I haven't amended the directions since 2000 & I
should do so. I would recommend higher CO2 injection rates (double) &
reduce the lighting recommendations to 1 wpg for low light plants & 2 wpg
for high light plants. I also would recommend clay balls containing N or NPK
fertilizer. I don't use a 3 layer substrate any more for myself; its
probably unnecessary with substrates shallower than 4".

For general use, I recommend a fairly mineral soil of low fertility with
about 5% by weight of dry peat; by volume, that's 1 part peat to 4 parts
mineral soil. Silt is best; you can find very wonderful mud from a river or
creek when the water is low. The Krombholz soil-soup
<http://www.thekrib.com/Plants/Fertilizer/soil-soup.html> method removes
soluble minerals (like nitrates) and the large organic bits from soil.

Whether to replace the substrate or not depends upon the value of the fish
in the tank & the risk of looking bad with your client. You say the tank is
several months old so I suspect that the worst effects of submergence have
already past. The best plants for providing oxygen to the substrate are
Bacopa & Alternanthera since they grow quickly given good conditions. Any
rooted plant will provide oxygen to the substrate even to the point where
substrate reduction processes decline. Having some organic material such as
peat, well mixed with a fine soil will probably provide local reduction
zones as Thomas has said that provide some reduced Fe, Mn & soluble
phosphate. I think you need a deep substrate before you can count on this
long term so chelated trace supplementation may produce an improvement after
a few years.

Steve P

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