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[APD] Barr Method Question



On Thu, 27 Nov 2003 06:14:14 -0600, Paul Krombholz <krombhol at teclink_net> wrote:

Dale wrote:
.....Even with 50 % weekly water changes and assuming no plant up-take (bad assumption I know), the nutrient levels would continue to rise in the tank.........

The nutrient level does not build up indefinitely. Assuming you add nitrate to initially give a value of 10 ppm, do a 50% water change, then add the same amount of nitrate, and that the plants don't remove any, the level in the tank will level off at 20 ppm. If you do as above, but with 75% water changes, the content will level off at 13.333 ppm. With 25% water changes you level off at 40 ppm. The general formula appears to be that you level off at the initial concentration times 1/(% water change)



I have a quick bit of iteration in Mathcad and the eventual level of nutrient in a tank, given infinite
water changes, and assuming no plant uptake, does seem to be given by:


		nutrient level added
	______________________________
	0.01 x percentage water change


(thats nutrient level divided by (0.01 times the percentage water change)).


The frequency of your changes is irrelevant to the final level of nutrient you will have AFTER each change
and fertilisation, but to avoid spikes change water regularly.


For example, if you do 50% water changes, and add 10mg after each change, then the eventual level will be 10/
(0.01*50), which is equal to 10/(0.05), which is 20mg.


Also, if you change less than 50% of your water, there is still a limit. For example, with 10mg of nutrient
and 25% water changes, it is 40mg in your tank at most (after a change).


I do not know how this is affected by a tank producing or removing a nutrient (e.g. fish make NH3, plants
take away NO3).


To avoid the slow gradual increase of NO3 in a new tank, would it not therefore be sensible to add the
maximum expected level as an initial dose, and then add the top up dose after this?


If anybody has any estimates of nutrient uptake/production rates as a function of concentration or similar, for non-planted/planted tanks, I could work out some figures (and maybe prove the Tom Barr 50% regime?)
--
Andrew McLeod
thefish at theabyssalplain_freeserve.co.uk
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