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Re: Aquatic Plants Digest V4 #1506

> If my calculations are correct, you are adding 2 mg/l of
> phosphate in each
> 1/8 tsp KH2PO4 to your 72 gal. tank.  Even assuming the level
> is zero after
> you do the water change, that seems excessive -- about 4 times
> what Tom Barr
> would recommend.  If you would like to check my arithmetic,
> please contact
> me off-list.

    Well it depends on what you want for a PO4 level. 1.0ppm
seems good to me personally(you may want to try something
different). I left town for a few days and loaded the PO4 up to
around 2ppm so that the plants would "make it through".
Consumption can be quite rapid, suprising even. My averages sit
around .2ppm to .3ppm of PO4 plant uptake per day of PO4. If I
starve a tank it will eat more. How long this starvation period
is in relation to plant uptake is not clear. I try not to push
the low readings,0.2ppm or less, for more than 2 days ever. Same
can be said for NO3 which has a greater effect on plant growth
than PO4. After a day or two growth slows at low readings.
    Something else to consider is all 1/8 teaspoons are not
created equal. I took averaged weights for KNO3 & K2SO4 based on
a level 1/4 teaspoon of 3 different teaspoon brands 10x for each
teaspoon. I used this amount(1.67 grams per 1/4Teaspoon) and
figured the SE(+,-0.11 grams) from there. Turns out that your
not far off based on brands of teaspoons.
I assumed that these are not such a big issue therefore when
measuring.But the smaller you go and the more precise you try to
be the more difficult it becomes. I have small spoon scooper
that averages about 200mg of PO4 per scoop. This gets me pretty
close to a target PO4 level for my tanks. But trying to shoot a
.5ppm target is much harder than hitting a 5ppm target don't you
   You can assume that the uptake is all plant uptake(algae or
otherwise) although laterite and some substrates will bind some
of the PO4. But after sometime this binding ceases to effect
things to any degree as it gets saturated with PO4 and cannot
bind any more. All that's left are bacteria and plants. No algae
presence->No uptake by algae. Now your down to bacteria and
plants. Bacteria do use small amounts PO4 but not like the mass
that plants do. There are not dephosphate bacterial reactors
around...I've never seen PO4 removal to any measurable degree in
any non planted tank yet(the control in the experiment). You can
see measrable PO4 when plants are there(plants being added as
the treatment). Therefore we can conclude that the plants are
the main users if there are no algae present. But if your
CO2/NO3/NH4 is not in the proper range? You get the other
plants..... algae.
        But a target level is flexible. Not to the same degree
that K+ is but if you go to 2ppm it's not the end of the world.
I know a few folks that run PO4 in that range. I've run it up to
1.5 ppm of long periods. Have not seen much reason to go above
that except for vacations:-) NO3, NH4, and CO2 are the ones that
cause most of the grief. PO4, K, traces can be added to excess
depending on if the other 3(CO2,NH4, NO3) are in good shape. If
these are in good shape, add more K, then traces and lastly PO4.
Go down in order, CO2,N(NH4 and NO3), K, Traces then finally
PO4. That should be your last nutrient to worry about in a CO2
enriched tank.
Tom Barr


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