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oxygen, fish, plants, light and CO2
This talk about oxygen levels and SAE deaths has started a train of
If you get unexplained fish deaths or diseases related to stress or
plants which turn brown and die, the cause may be low levels of oxygen.
One way to approach this problem is to get an oxygen test kit which are
widely available in the more upscale fish stores. George had another
high tech gadget, I think, which essentially measured oxygen content via
the redox potential. For anyone interested in a soil/peat substrate it
would be a good plan to plot oxygen levels during the first few weeks to
gain some valuable data.
Another way to approach this problem (without test kit) is to take a
look at all the factors which affect the oxygen level in the water and
how to modify them. If you are injecting CO2 and you are minimizing
surface disturbance to conserve it you are also reducing the interchange
of oxygen in the air with the water. If you aren't using CO2, then you
want to have the water circulating as much as you can to let the plants
get at the CO2 available in the water. Even if you are adding CO2, its a
good plan to get the water circulating (I like powerheads for this)
because this maximizes CO2 availability to the plants and it does
improve the gas exchange rates at the water surface in case you don't
have enough photosynthesis happening to keep the water oxygenated.
When things are working well, at the end of the day, if there aren't too
many fish, (or other consumers of oxygen like bacteria rotting things),
you will often see the oxygen pearling. Sometimes you can observe that
at the _end_ of the day, it looks like your CO2 bubbles aren't
dissolving as much as they swirl out into the tank from your pump. This
isn't actually because the CO2 isn't dissolving; just the opposite!! It
is dissolving however oxygen is diffusing almost as fast INTO the
bubbles (as mentioned by Roger Miller in the past).
No oxygen pearling, stress problems with fish or plants? Ok, maybe you
can reduce the oxygen demand. That could be by removing fish, snails,
detritus, wood, even peat. Of course you can't always remove organic
matter from a substrate very easily but you can sure keep the fish load
low during the break in period and you can avoid using soil that is high
in organic material. Soil that is good for your garden, may be VERY BAD
for your aquarium. In fact, you can get iron toxicity which is something
which can turn your plants brown and kill them such as the incident that
Matt MacGregor described in 7 Aug 98 APD # 434. Iron toxicity happens in
rice fields when they grow it on a newly submerged soil. That's because
this fresh soil contains enough organic material that the bacteria use
up most of the available oxygen and nitrates and the soil goes "sour".
Aquatic plants growing in swamps need to produce a LOT of oxygen to
protect themselves from high levels of reduced iron and ammonia and
other reduced compounds which potentially can harm their roots. For most
kinds of aquatic plants, this is no problem so long as they have an
adequate supply of 1) light, 2) CO2 and 3) the rest of the nutrients.
Aquatic plants are able to carry oxygen into their roots through special
air channels in the stems and roots called aerenchyma. Plants with thick
roots seem to be bested adapted for this. Some of the best include:
Bacopa, Myriophylum, Alternanthera, Elodea, Echinodorus and Crypts. In
my experience, the fast growers like Hygrophila and Rotala seemed to be
at a disadvantage especially after being trimmed or freshly replanted
however they are the second wave of colonizers in a new tank.
Why do the high peat substrates not suffer from iron toxicity? My guess
is that a lot of these substrates may not have a large amount of
available iron (such as if you added micronized iron) or nitrates which
might be reduced to ammonia or other fertile substances which can be
reduced to toxic forms. The other scenario could be that the aquarist
may be using plenty of "break in" plants and providing good conditions
(light, CO2 etc.) which are producing lots of oxygen and keeping things
I think we really don't need very much peat or organic matter to keep
iron available in a substrate. It would be nice if we could add the peat
at a later stage in the evolution of the bio-system when the plant roots
were really well developed and pumping too much oxygen into the
substrate and depriving themselves of a source of iron. Or maybe the
smart thing to do at that stage is to just to switch over to chelated
Morale: the safest approach with a soil substrate is to use a mineral
soil (very low on organic matter). This means skipping the peat or
perhaps only using a small amount such as a cup or two for average sized
tank. I believe I will adjust my recommendations about peat. I notice
that some the commercial aquarium and pond soils contain relatively low
amounts of peat however they do use it.
For more experienced aquatic gardeners, using the 3-5 watt/gal lighting
and using compressed CO2, you can probably push the organic load up
without much risk.
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!!!