I view test kits radically different from most hobbyists, namely because they are tools to investigate hypothesis for myself, rather than monitoring.
Thus they are very good when used properly and can teach you a lot and answer many questions.
I like folks to prove things for themselves.
Often, I hear the failures, folks who could not follow the directions or had something go wrong they could not pin point.
They blame a method for that, rather than themselves.
They often cite test kits in support.
But these same folks also do not calibrate nor moniter things well to begin with and cannot produce a control tank to manipulate.
I started off being pretty good, not really testing.
Then I started testing a lot.
Then I answered many questions I had.
Then I realized I did not really neee dthe test kits any more.
I also realized that most aquarists, if they chose, and the trade offs are great, also do not need them.
But if you want to prove some things, investigate etc, then yes!!!
I think everyone that has ever taken my test kit advice has been very pleased once they bought the $$$ nice kits.
I have a 3000$ HACH spect to do much of my testing.
I was not as pleased with the Hanna as I'd hoped(still an 800$ unit).
I have 2 light meters that are waterproof that measure in Micromols/m^2/sec.
I have a DO meter
I have 2 pH meters
I have an ISE probe based meter
I have a redox and temp meter
I have a 0.001 gram scale
I have a custom made pH/KH reference CO2 meter that accurate to about 0.1 ppm CO2.
And at the lab we have a Beckman spect, Atomic adsorption, all sorts of meters, methods, tissue anaylsis, you name it.
Yet for all this equipment and methodology, I still generally suggest folks to not really test, rather, watch the plants carefully.
I've come full circle several times with all this and added the human factor into this and my advice.
They(the plants) are ultimately what you add/not add things for, not to achieve some water parameter criteria, that's just a ballpark "range", not some precise measure.
Regards,
Tom Barr