The Kenosha Kid wrote:

I switched to a Hagen Nutrafin test kit with several different tests
in the same kit. It has a test for NO3 which I used to cover using a
Tetra test kit.

I ran tests for NO3 using both kits on samples from the same body of
water. The new (Nutrafin) kit gave me a reading at or below 5 mg/l
while the Tetra kit gave me a reading at or excess of 50 mg/l.

The Hagen kit is infamous for reading low. The problem is the third
reagent, which you must shake and dispense without allowing particulate
matter to settle. This same particulate matter will often clog the
nozzle, rendering the kit useless. Hagen will replace the third bottle
with a glass bottle, but one wonders why they have been selling this kit
for years with the same problem.