I'm a little confused, too. How is the nitrate from supplied doses of KNO3 any different or better for the plants than naturally occuring nitrates in the water from fish food and fish waste? I can't imagine it is. Test kits, I don't believe, are selective as to which nitrate it is registering -- the kit measures the total nitrate in the water (nitrate from food, nitrate from fish waste, nitrate from Greg Watson KNO3). Someone please do correct me if I'm wrong. I'm probably simplifying things.
As for the test kits going bad, I did "control" test them against my tap water and a few controlled nitrate dosed water samples of varying amounts. Everything seems to be in check, especially the Lamotte kit.
When I first set up my tank months ago, I did the occassional water test, more out of curiosity to see how the system was working and what my plants were consuming. I had less fish in my tank then, lots of stem plants, 192w on for 11 hours, and tons of Water Sprite. Using my Lamotte kit, it was amazing to see just how much NO3 the plants would suck up in just 2 days. They consumed nearly all of it. I dosed "blindly" from that point on: no testing, just EI dosing and a water change.
So why did I decide to test recently? I cut down my wattage. I added a lot more fish. I fed a lot more. I removed all of my Water Sprite and nearly all of my stems. My CO2 was still strong but I seemed to be getting faster dust algae on the glass and more spot algae. Decided it was time to test the water. That's when I saw high NO3 and PO4, even after water change.
15ppm of NO3 and 2+ppm of PO4 is enough for my tank. No need to go higher. And since backing off supplemental macros, I've had less dust algae on the glass and less spot algae. For my tank, those levels seem like the sweet spot. I tested again today -- if anything, my NO3 went up a point. Water change time tomorrow. I will check again next week. If my fish, feeding, plants, and light stay constant I will only dose micros for a while and see what happens.
Yes, the beauty of EI is not testing. I like that. So hey, if I don't have to dose water column macros either, even better.