I recently bought and tested five inexpensive nitrate test kits.
I am not a chemist. The tests were conducted in my kitchen. My eyes are not capable of discerning some of the subtle differences between colors on the color charts that accompanied all of the kits, if in fact the colors printed on those charts accurately portray the colors that should result from tests, a big “if.” Nonetheless, I believe that the tests accurately portray the differences between the kits.
One other problem is that I don’t know what the “right” answer is for the three test solutions that I used. For example, my tap water comes from a 150 foot deep well in an area with very little agricultural influence. I don’t know what the nitrate level is in the water from that well, but four of the five kits measured about the same amount. The fifth didn’t measure any. Since that fifth kit also reported substantially lower levels of nitrates on the other tests, I am inclined to give more credence to the other four, but others might make a different choice.
One additional note: Since one of the kits produced substantially different results than the other four, I am inclined to question its accuracy. But to be fair to that vendor and to respect the interests of the management of this forum, I will not identify it here. If anyone should want to know the identity of that vendor, he or she may email me and I will respond.
Test 1. Tap water, as described above
API, 11 – 12 ppm; Jungle, 10; Tetra, 13 – 14; Wardley, 11 – 12; Brand X, 1
Test 2. A solution of NO3 and tap water:
API, 120 – 150 ppm; Jungle, 50; Tetra, 20; Wardley, 90 - 100; Brand X, 0
Test 3: A healthy, heavily planted, soil-based aquarium, set up for several months:
API, 1 – 2 ppm; Jungle, 5; Tetra, 2; Wardley, 0; Brand X, 0
In summary, it seems safe to conclude that four of the five give reasonably accurate readings at commonly encountered nitrate levels, but that they diverge significantly when measuring very high levels.