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Re: Biological Filter in planted tanks

Thanks to all of you who got involved in this topic. It does seem that quite
a few people are running their tanks with minimal biological filtration
quite successfully. Some very interesting points were brought up. Tom Barr
mentioned that he recalls reading (or hearing) that plants use 25% NH4+ and
75% NO3- to derive their Nitrogen requirements.
www.hydrofarm.com/content/articles/factors_plant.html also lists this same
ratio (together with references) but also brings up the subject of ammonium
toxicity (another topic). Roger Miller brought up the point that nitrogen
uptake in plants might happen both during the light and dark cycles. I agree
with Alysoun's opinion though, that the plants ability to uptake any N would
be inherently limited by the availability of other nutrients and hence if we
have an imbalance, ammonia might indeed shoot up.
And now lets get back to the biological filter. It is still my opinion that
plants and nitrifying bacteria WILL 'compete' for the available ammonia.
First, lets look at a simple case study. Assume that we have a bare tank
fully cycled (this would mean we have some sort of ammonia source like fish
waste). Fully cycled would actually mean that we have large amounts of
bacteria capable of reducing all the ammonium to NO2 then to NO3. Both
bacteria (nitrosomonas and nitrobacter) are lithotropic requiring oxygen and
their food source (NH4 or NO2) to survive and multiply. If either one of
these two are limited their colonies will die back to a new point of
equilibrium. Testing for NH3/NH4 or NO2 in this hypothetical tank will read
0 ppm. Now what happens if we introduce plants to the tank? The outcome is
dependant on a number of variables including the ability to get to the
ammonia first (is it the plants or is it the bacteria?), limiting factors
(like nutrients for the plants or oxygen for the bacteria. Bacteria will
also be limited by the available surface area in the tank), water
circulation (the ammonium ions would have to be in direct contact with the
aquarium surface for the bacteria to be able to use it, the same would apply
for the plants), etc. It is feasible to assume that the plants will get to
some of the ammonia (especially if we do have a slight water current to mix
things around) which should cause some of the bacteria colonies to die back.
Eventually a state of equilibrium should be reached which again will be
dependent on the exact conditions we create. This is also the opinion of
most of the respondents to my original post (Cathy, Tom, Roger and Alysoun).
I would go as far as saying that adding media that offers a large area for
the bacteria to colonize and then running water over it would not be in our
best interest (if we are trying to grow plants that is). Of course, as
Alysoun mentions, there will be a point where the bio load is so high that
neither the plants nor the available colonization surface is enough to
'filter' out the ammonia and one or the other needs to be increased (Tom is
suggesting more plants, another option would be filter media).
There is a second case to consider and that is starting a new tank fully
planted up before any bacteria take a foothold. Will the same equilibrium
point be reached as in the first case or will it be different. I'll leave
that for the next post.

Adrian Banica
Edmonton, Alberta