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LaMotte Kits, phophate-limited question

Frustrated with several test kits and pleased with the 
LaMotte CO2 kit, I ordered their Iron, Nitrate and Phosphate
Kits. Note that they make two different phosphate kits. 
#3231 has a range and readings of 0.0, 0.2, 0.4, 0.6, 0.8,
1.0, 1.5, 2.0 ppm. LaMotte #7416 has 0.05, 0.1, 0.2, 0.3, 
0.4, 0.6, 0.8 and 1.0). They use different test methods 
(ascorbic acid for the #3131 and stannous chloride for 
the #7416 I think). The #7416 is not on the LaMotte website
yet. I saw it in a That Pet Place catalog. 

I chose the 0-1 ppm range kit, since SeaChem says to change the
resin when the phosphate gets to about 0.1 ppm.  Also, I figure
that to test my tap water and it is between 1 and 2 ppm I can 
dilute my sample with distilled water. (There is a mistake on 
my LaMotte comparator: the 0.4 reference is marked 0.1). 

I am starting to add 0.5 M KNO3 to make the tank phosphate
limited. My reading is about 1 ppm nitrate (converting from
from nitrate-nitrogen). Here is my question. Is there an 
optimal nitrate value that I need to determine and maintain,
or is any low reading, say 5-10 ppm OK?

BTW, while the Tetra nitrate test was useless for me, I
find their KH and O2 kits very good.