Re: More musing on water change.

["Tyrone Genade" <Tgenade at sun_ac.za>]

On 25 Jul 2002, at 10:08, Wright Huntley wrote:

The biochemist to the rescue... perhaps.

> Charles n Sue Harrison wrote:
> > There is no equilibrium. and that is a common thought, that there
> > must be.  

No. And I will show why this is wrong but only after I give Wright a 
good flaming... (how rare)


> OK, Charles. Can you show me the arithmetic that demonstrates that?
> 
> If the added poop/concentration each week is *less* than that
> contained in 25% of the total volume, you removed more than was added,
> with a 25% change. How does that eventually reach a solid sludge
> stage? Sounds like a classic partial dilution to me.

The fish produce for arguements sake 10 mg/ml of waster per week. X 
does a 25% water change. Now there is 7.5 mg/ml waster left. One week 
later and now there is 17.5 mg/ml waste. 25 water change and then 
there is 13.125 mg/ml... eventually you will reach a situation where 
the tank is a puddle of sluge. No. Plants and bacteria will take the 
waste up and turn it into biomass which is siphoned out from the 
gravel or rinsed from the filter medium. But still there is NO 
EQUILIBRIUM this way. So far Charles is right. 10 pts to him, 0 for 
Wright.

> Tom's recent post about his flow-through system is a classic case of
> partial changes (sump only) doing an excellent job. The filtration
> takes care of much of the rest and his baby-fish growth proves it is
> working.

If you have a 100 l tank and drip 1 ml of water into the tank every 
minute for 100'000 minutes you will only have a 99.999.... % water 
change. You can never flush the tank 100% unless you remove all the 
water and fish and let the tank dry.
 
> > There is only one way to get back to the start . . . Change it all.

Yes! another 10 pts for Charles.
 
Now about equilibrium in tanks... As you always have to add food you 
never reach an equilibrium. But, if you have a tank if algae eating 
platies in a big planted tank once the initial energy investment to 
grow algae to feed the plates and grow the plants you can reach an 
equilibrium but this depends on the CARRYING CAPACITY of the tank. 

[Interresting side note: see the article "Tracking the ecological 
overshoot of the human economy" PNAS 99: 9266-9271, 
http://www.pnas.org/cgi/doi/10.1073/pnas.142033699 for a good time. 
It states that humanity is in excess of 160% of the earths carrying 
capacity. There working is abit iffy but the article remains an eye 
opener.]

Diana Walstad, in her fabulous book (best buy I ever made), explains 
how to reach this balance where all you do is add food to the tanks 
which I think is what everyone wants. She does say that she does 
regular maintenance and does do water changes every other year...

Strictly speaking there is no equilibrium in the standard fish tank 
and eventually one will have to do a bigger water change or clean the 
tank as waste builds up.

tt4n

Tyrone Genade
tgenade at sun_ac.za
http://www.tyronegenade.0catch.com

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