Why can't everyone just use the damned metric system. Fuck. Why do we have to be 'different' with our ooooh fancy base-12 BULLSHIT. Dammit. Determining %v/v with units like teaspoon and gallon makes my head hurt.

Okay hang on one sec. H3PO4=mw 98.0
1 tsp=4.93mL call it .005L
5 gal=18.93L call it 20L (proportionally the rounding works)

Checking my acid storage cabinet, I see that we've got it as an 85% solution, standard for packaging it in concentrated form. At 85% standard concentration, it's 15.2 Molar, 45.6 Normal. Damn that bottle is heavy! And DAMN is that a high normality... shit's tri-protic... DON'T spill any, hahaha!
Okay back to chemistry...
15.2m/L = Xm/.005L
X=.076
Your teaspoon contains .076mol of the acid.
H3PO4 weighs in at 98.0g/mol
98.0g/1mol=Xg/.076mol
X=7.448g.
Now the step that is easy to forget, of that 7.448, how much is Phosphorous?
The molecular weight of P is 30.97 g/mol. There's only one P in the molecule.
30.97/98.0= 31.6% phosphorous.
Your teaspoon contains 2.35g of P.
2.35g/20L for a %w/w in water is 2.35g/20,000g
Doing this as %w/w, your %Phosphorous from the pH down you are using is .000177%...
No need to fret, it won't interfere with your feeding schedule!

This from Kathleen Carroll:
[align=center]Refrain:
Oh, metric. You??re the one.
You make measuring more fun.
Based on ten. Let??s start again
And go the metric way.[/align]
[align=center]There was once a family of giants
And Kilo was their name.
They were all so huge and tall
And no two were the same.[/align]
[align=center]Now, Kilogram, he weighed a lot
And Kilometer was long.
Kiloliter was big and wet.
Each one was a thousand strong.[/align]
[align=center]Refrain[/align]
[align=center]There also were some tiny elves
Who had the name of Milli.
One one thousandth??they??re so small
To think of them seems silly![/align]
[align=center]Refrain[/align]
[align=center]Now milligrams, they don??t weigh much.
And millimeters are short.
A milliliter??s a tiny drop
Of liquid they report.[/align]
[align=center]Refrain[/align]
[align=center]So, grams you may have noticed
Are things you can weigh.
Length is meter, liquid??s liter.
That??s the metric way.[/align]
[align=center]Refrain[/align]