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.

I repeated the tests twice and got the same results.

Apart from the Tetra kit being a year old are there any other reasons
for these results to be so far apart?