I might try the Salifert NO3 test kit. My tank is 37 gal, hi-tech with pressurized CO2, admittedly overstocked, and very heavily planted. The API test kit showed my nitrates consistently in the red, so I calibrated it using distilled water and set amounts of NO3. The API test did not show any difference between 50 and 25 ppm (both were the same red). 10 ppm matched the orange color on the card, but 5 ppm matched the yellow color for zero ppm. So all it really tells me is I'm somewhere between zero and 20 ppm if the result is orange, and somewhere above 25 ppm if the result is red.
I dry dose EI ferts, using 1/16 tsp K2SO4/KH2PO4/CSM+B, and 1/32 tsp KNO3. My nitrates stay higher due to the bioload. I've experimented with not adding any NO3 to the tank. While it reduced my nitrates into the orange over the week, I noticed the plants were lacking NO3 (yellowish color, slow growth, some brown algae). So I add a small amount of KNO3 on the macro dosing days. My nitrate level stays in the red, but the plants grow well and my fish/shrimp are active and seem healthy. I just have to supplement with K2SO4 since I reduced the normal EI amount of KNO3 (to prevent running low on K).
So I agree with the people who say to focus more on visible plant/fish health than test results, especially if you're using the API nitrate kit. I do want to be sure the fish are in a healthy environment, though, which is why I might get the Salifert kit. From what others have said, it will probably show a much lower nitrate level than the API, anyway.