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Re: Fish load .v. plant load

On Wed, 19 Jan 2000, Kevin Buckley wrote:
> What I would like to understand is whether, if I were to increase my light &
> add CO2, I could ever reasonably get to the point where the plants eat all
> of the ammonia produced by the fish?

It's possible.
> Has anyone ever done a rough quantitative analysis of this? i.e. the
> relationship between "fish-food-in (grams/month), plant-material-removed
> (grams/month) & rate-of-Nitrate-change (ppm/month) given a particular water
> volume & static fish mass increase (i.e. Nitrogen-into-fish =
> Nitrogen-out-of-fish)?

I've done it, but nitrogen isn't independent from other nutrients in the
tank so I don't think it can be done in a meaningful way without also
addressing at least carbon and the other macronutrients at the same time.  
It gets complicated and it's real usefulness is questionable.


change in nitrogen content of the tank = nitrogen in - nitrogen out

the nitrogen content in the tank includes the nitrogen in the water, in
fish, plants, mulm, snails and so on.  It's very difficult to quantify.  
It's simple and at least semi-realistic to assume that there's no change
in the total nitrogen content of the tank.  This probably describes a
mature, carefully maintained tank.

That leaves (after rearranging):

nitrogen in = nitrogen out

To simplify things assume that you don't fertilize with nitrogen,
and that there's no nitrogen in the water you use for water changes.  Fish
feeding would be the only "nitrogen in".

nitrogen out is more complicated; nitrogen is removed with plants and
trimmings, with filter cleaning, with water changes and with
denitrification. If the tank isn't filtered and you neglect
denitrification then things are simpler.  The nitrogen removed by plants
is the plant dry mass removed times the nitrogen content of the plants.
The nitrogen removed in water changes is the nitrogen content in water
times the volume of water that is changed.


feeding rate*nitrogen content of food ~=
  plant removal rate*nitrogen content of plants +
  concentration in water*water change rate.

> My fish food is 48% protein, & 100g lasts me about 4 months - does anyone
> know how much Nitrogen that represents?  What is the Nitrogen content of
> 'wet Vallis'?

Proteins (collectively) are about 16% nitrogen.  To get the nitrogen
content of the fish food, just multiply the protein content by 0.16 (or
divide it by 6.25 -- same thing).  48% protein translates to 7.7% N.
100g of fish food then would contain 7.7 grams of N.  Over 4 months,
that's 64 milligrams/day.

If plants are really nitrogen starved, then their nitrogen content might
be as low at 16 mg/gram (this is from the "Critical Concentration" given
by Diana Walstad).  At that concentration, your 64 milligrams/day of
nitrogen in fish food would produce 4 grams of dry plant mass per day.

Aquatic plants are mostly water.  I don't know what vals are, but I seem
to remember that the normal number is something like 95-97% water by
weight.  If the plants are 95% water, then 4 grams of dry plant mass per
day would translate to 80 grams of actual plant mass per day.

Your plants aren't nitrogen starved, so their nitrogen content should be
much higher than the critically low value.  The approximate concentration
that Karen Randall gave in one of her AFM articles was 1% to 3%; Diana
Walstad reported that a mixed population of plants from her tanks (low
tech and not nitrogen limited) was analysed to contain 3.9% nitrogen.  If
your plants ran about 3% nitrogen (30 mg/gram) then the nitrogen in the
fish food would be contained in 2.13 g/day of dry plant mass, or about
42.7 g/day of wet plant mass - close to 300 grams/week.

If you can grow 300 grams/week (or so) of plants, then the plants can
contain all of the nitrogen you add with the fish food.  300 grams/week is
about 11 ounces per week

Assume your feeding rate of 25 grams/month and feed nitrogen content of
7.7%.  Say you're removing 60 grams (60,000 milligrams) of plant dry mass
per month, that the plants contain 3% nitrogen, and that you chang 38
liters of water every two weeks, or 76 liters/month.  The approximate
nitrogen balance based on the formula back at the beginning of the letter

25 grams food/month*0.077 grams N/gram food = 
   60 grams dry plant/month*0.03 grams N/ grams dry plant +
   C (grams N/liter) * 76 liters per month.

This lets you (through the magic of algebra) estimate the expected
concentration of nitrogen in your water.  For these numbers it comes out
to 0.0016 grams N/liter, of 1.6 mg/liter N.  That would be about 7 mg/l of
nitrate.  In reality it probably would be lower than 7 mg/l.
> When plants uptake Nitrogen do they preferentially take Ammonia rather than
> Nitrate or do they take both simultaneously if both are present?

Most aquatic plants seem to take ammonia by preference but will take
nitrate if they need it.  In a mixed community of plants you're likely to
have some that use ammonia and some that use nitrate.

> Also, do, fish faeces biodegrade into something other than Ammonia (related
> to the previous point)?

Fish feces degrades to a lot of things other than ammonia. Most of the
nitrogen content of the feces should be in amino compounds, including
undigested proteins.  Some of these compounds will be broken down by
bacteria to release ammonia but some of it will be pretty refractory; it
won't break down quickly - in fact it may never break down - and it will
end up as a permanent, benign nitrogen content in the mulm.

Fish release ammonia through their gills and urine.  Ammonia derived from
decomposing feces isn't the only source of ammonia in an aquarium.

Roger Miller