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Re: Phosphate/nitrate chemistry. A puzzle?

>                                 Regardless, if phosphate is generally 
in excess in fish food, compared to nitrate, I can't figure 
> out why both tanks don't accumulate excess phosphate. Does anyone have 
any ideas?

I love puzzles :-).

> The tanks:
> 75 - (High nitrate, low phosphate tank)
> Fish load heavy: 12 juvenile angels, 18 cardinals, 4 siamensis, 4 1.5 
inch "checkerboard ciclids, 8 Otos;
> 29 (low nitrate, high phosphate tank)
> Fish load: two adult angelfish (about 3 inches long), one spotted Raphael 
(about 2 inches), and 2 Otos.
> rdenney at ibm_net
> Richard M. Denney

So, are there plants in these tanks?  I'll assume that there are and
that here is enough plant or algae growth in both tanks to influence the
water chemistry and that both tanks contain the same (or at least
similar) assemblages of plants. 

I've read a bit about N:P ratios in lakes and there tends to be a lot of
variation in their behavior.  As a result, I imagine that their behavior
will vary from aquarium to aquarium, as well. 

Generalizing grossly, there is an N:P ratio below which N tends to limit
plant growth and above which P tends to be limiting.  N (ammonium and
nitrate primarily) usually stays in solution or gets included in tissue
while P (hydrogen- and dihydrogen-phosphates, mostly) tends to get
partitioned into solids; adsorbed to metal hydroxides or bound up in
insoluble precipitates.  Some is also included in tissues or remains in
solution.  Because of these different behaviors, the N:P ratio in
solution is usually much higher than the N:P ratio in the material added
to a tank and most often P becomes the limiting nutrient.  Which ever
component is limiting, its concentration gets driven down to low values. 

In the 75 gallon tank, P is probably limiting, as it is low.  In the 29
gallon tank, N is probably limiting.  So why the difference?  One 
possibility is that the juvenile fish in the 75 gallon tank need
more P than the adult fish to support their growth.  So the N:P ratio in
their wastes (and hence in the rest of the tank) is higher in the 75
(with the juvies) than in the 29 (with the adults) and the small
difference that creates is great enough to make P limiting in the 75 and
N limiting in the 29. 

Of course, if iron or CO2 or some trace were limiting, then this would 
all be a crock.  It would *really* be off if there's no plants.

Roger Miller