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re: the great eqm debate

Hello again

The situation described as the fishtank-fishkeeper eqm is more 
correctly a steady-state. A homeostatis is attempted (but ultimately 
fails due to the complexiity of the system) as opposed to an eqm 
where "balance" is acheived.

Quickie definition:
Equilibrium (Eqm): A + B <=> C + D, the rate of C + D formation 
equals the rate of A + B formation from C + D. If we let A be food 
and B be fish ther is no way that food and fish can be formed from 
waste (C) and biomass (D) in our fishtanks.
Steady-state: A +B <=> C + D <=> E +F where the rate of C + D 
addition is supposed to equal the rate of C + D removal.[A = food; B 
= fish; C = NH3; D = biomass; E = removed biomass/bacterial sludge; F 
= NO3-/PO4-3 + H2O; we can perhaps also add the term H alongside C 
and D which represents the hormones (good and bad) that are not 
converted to anything.]

As we don't really know how much C + D is formed we will iether over 
or under shoot the rate of removal needed. Also, we never get all the 
"E" and "F". 

A good example of an overshoot is the maintenance of chemostats where 
in bacteria/yeast (bugs form this point on) are grown. As long as the 
dilution rate equals the growth rate of the bug a constant density of 
bugs is maintained. Increase the dilution rate and the densuity drops 
(but never to zero!), decrease the diltuion rate and the density will 
increase to the point of the new carrying capacity of the system.

So, fishtanks can't reach equilibrium and they will not reach a 
steady-state for the simple reason that they are not as simple as 
chemostats where that "simple" math works vey well (the math is not 
simple FYI!!!).

A shorter interval between water changes will cause an decline in the 
waste but never to zero.

How many fish you have in the tank determines your frequency of water 
changes. If you have a small number fish (biomass) you will need less 
drastic action, the more fish (biomass) the more drastic the action 

Going back to the De Bruyn filters, zero "waste" readings are 
achieved without water changes so all the waste is gone. No, the 
waste is in the form of the bacterial sludge at the bottom of the 
tank which De Bruyn says he removes periodicly. If this sludge was 
left to ferment there would be an explosion of NH3, NO2-, NO3- and 
De Bruyn hasn't reached any equilibrium at all with his filter he is 
still driving A + B to C + D and eventually C + D will have to be 

Not even Diana Walstad's tanks are at Eqm. She feeds the fish and 
trims the plants. She quite possibly has a steady-state going. The 
tanks adapts to the influx of energy by stimulating bacterial, algal 
and plant growth that remove the added energy (fish food) but Diana 
still has to go and trim the plants back or else they are just going 
to add to the problem eventually and stop being the solution. (I 
suggest that is you are going to debate this point further you 
consult the work on Metabolic Control Analysis by David Fell, Athel 
Cornish-Bowden, Jannie Hofmeyr etc... for some clue as to the 
complexity of the math and its application.)

If you want to create a tank at eqm get a really big tank. Put in 
sand, plants, bacteria, algae and a few algae eating fish, snails and 
perhaps some shrimp. Seal the tank air-tight! and just give it light. 
No matter enters the tank, only light energy. Perhaps over a long 
time and much internal instability an eqm will form. I have every 
intention to try one day...

I recall an article in FAMA some years back of a Prof setting up 3 
large tanks and letting them go as natural as possible. Eventually 
the tanks needed extra nutrients in the form of NH3, NO2- and NO3- as 
well as water top-ups. No eqm was acheived, not even a steady state...

Tyrone Genade
tgenade at sun_ac.za

P450 Lab, Biochemistry Department
University of Stellenbosch, 7602, South Africa
Ph: +27-021-808-5876, fax: +27-021-808-5863

"Seek your happiness in the Lord."
                  Psalms 37:4
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