If you insist on wasting time and money testing your water using test kits, then proper "big boy" kits are the way to go.
That's a pretty broad statement
When I did a fishless cycling of my new tank, the API ammonia test kit showed concentrations consistent with the amounts of aqueous ammonia I had added to the tank. Subsequent nitrite measurements were consistent with the stoichiometry. Threadfin Rainbowfish added after the test showed zero nitrites did well. The tests apparently served their purpose.
As for the nitrate test accuracy, I appreciate the alarms raised here. I
would like an idea of actual nitrate concentrations, and might get a Hach kit if I find that API testing of samples of my (C-filtered) water with measured amounts of my fertilizer solution gives inconsistent results. Don't need absolute accuracy - approximate should do. The Hach test, at $1+ per test, might be overkill for routine monitoring.
Colour kits work by nitrate reduction to nitrite (NO2-), which forms a number of coloured compounds. The two normal methods are <
"cadmium reduction" and "azo dye" formation">. Cadmium reduction has the <"
cadmium (Cd) issue">, so the "azo dye method" is usually used (a red colour shows the concentration of NO3, with cadmium reduction you get an amber colour reading). If you have an amber coloured kit, you really need to shake the bottle hard to make sure you get the full colour development.
The pic shows an older API nitrate test bottle #2 that I cut open to have a look inside. I assume the silver-grey residue on the bottom is metallic Cd powder. It's loose - not caked. I taped the bottle closed, shook it vigorously, and found most of the powder in suspension when I reopened the bottle.
A bit of reading of "API nitrate test reviews" indicates that test results are technique-sensitive, which is probably the case with all hobby tests, and perhaps more-so with this one. I suspect that shaking of the final sample-reagent mixture for the time specified is probably more crucial than prolonged shaking of reagent bottle #2 before dispensing - as long as a surplus of Cd is dispensed for the test. I calculate the Cd equivalent of 40 ppm nitrate, e.g., in a 5-ml sample to be 0.36 mg.
If I'm going to check the accuracy of the test with my water, I might as well check results with varied shaking to calibrate my technique.
I'll also get a conductivity meter for additional information and will probably use it eventually for monitoring. At this point, however, I'm not sure that conductivity alone will tell me how much nitrate I have, versus an accumulation of other fertilizer components such as sulphates.
And I'll read up on your Duckweed Index
Thx.