Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
I use API test kits. The nitrate test has worked fine for me, even well past the expiration date it shows no signs of failure when tested against calibration solutions. As long as you shake bottle #2 very well that is. The active ingredient is solid crystals in liquid suspension, which tend to settle and clump together over time, and if they're not well mixed you'll get bad tests. Rapping the bottle on a hard surface helps break up clumps too, prior to shaking.
I tried upgrading to a fancier and more expensive Red Sea nitrate test once. Which always produced a result of zero. It was replaced by the manufacturer, but the new one had the same problem. At which point I got to speak directly to a chemist working for Red Sea, and during troubleshooting I learned about all the ways nitrate tests can go wrong. None of which got the Red Sea test working, but ironically the technique he gave me (described above) got my API test working perfectly. So I went back to API.