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Re: Phosphate

In response to a posting from someone who was adding phosphate to her tank,
I wrote:

> 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.

Thomas Barr responded:
Date: Tue, 1 Jan 2002 11:21:21 -0800 (PST)

> 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).

The reason I thought the target should be about 0.5 mg/l was because of an
e-mail you sent me on Dec. 12 suggesting "Try for PO4= .5ppm "  I guess I
assumed if you said it, it was carved in stone!  Sorry, didn't realize it
was flexible.

>     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.

It was that 1/8 tsp that caught my eye, though I didn't know about teaspoons
varying in volume.  When I did my calculations, I poured the whole 500 g of
KH2PO4 I bought into a big measuring cup. It came to about 1-2/3 cups or 80
tsp. That's about 6.25 g. KH2PO4 per tsp. or 4.4 g. PO4 per tsp. In my
30-gal. tank, a tsp would add about 38 mg/l of PO4.  It seemed impractical
to even think of trying to use any fraction of a tsp for the job.  So, I
dissolved a tsp in a liter of water, which gave me a solution containing 4.4
mg/ml PO4.  To get a 0.1 mg/l increase in 30 gal. (114 l), I would need 11.4
mg of PO4 or 2.6 ml of solution.  That's a quantity I can measure easily.

My plants seem to be using up about 0.15 mg/l per day, and though I don't
know how accurate my Seachem phosphate kit is, it does appear that about 4
ml of solution brings it right back up.

> 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.

Will keep that in mind.

> Regards,
> Tom Barr
John T. Fitch
E-mail: jtfitch at spamcop_net
Web Site: www.fitchfamily.com/aquarium.html