So I decided to get into this planted aquarium business with my 20g
tank with several Crypt wendtii and Anubias nana barely hanging on for
a couple of yeas now due to low light (half-dead 20W fluorescent),
lack of CO2 and fertilization (all other plants died off). I started
by adding a 55W power-compact from AHsupply and some fertilization
from Seachem, added some fast-growing plants, and have the
Eco-complete substrate and CO2 regulator on order. No need to mention
that immediately I got a huge algae bloom ... but that's not the
question I wanted to ask now.

I bought two nitrate test kits: one from Red Sea which was rated as a
"decent middle-end" kit, and - for comparison and a peace of mind -
another one from Aquarium Pharmaceuticals. Now, instead of peace of
mind, I have a headache. At the first measurement, the Red Sea test
gave ~8 ppm NO3 (between 5 and 10), while the AP test - 80 ppm! One
week later, the Red Sea shows 2.5 ppm, and the AP - 20 ppm, and
finally one week later - 0 ppm on the Red Sea test, and 5 ppm on the
AP. Tap water measures 0 ppm with both tests. So it seems that there
is a 10 times difference between readings (in the last measurement, 0
ppm on the Red Sea test could be as well 0.5 ppm). And yes, I followed
the instructions precisely and used the correct scale.

Now the obvious question is which test kit should I believe? The real
question, however, should be what is the reason of such a huge
discrepancy between test kits? Red Sea test kit uses 3 reagents, while
AP test kit - only 2 reagents, so they probably use different chemical
reactions. Could it be the interference from some other chemicals in
the water, such as phosphates (I have high phosphates at 1.5 ppm which
won't go down, in contrast to nitrates)? Anybody has any ideas?

Thank you,
OK