those results could be analogous if one test gives results "as Nitrogen" and the other "as Nitrate" (along with a bit of error).
Sorry, both are Nitrate tests reading concentrations of Nitrate ion.
One kit is the API nitrate test, which reads 20ppm on my municipal tap water. It reads ppm of nitrite (NO3-). I've tried many times, shaking both gently and as hard as I can for the time periods prescribed in the instructions and have consistently gotten 20ppm for my tap water. It does correctly read distilled water at 0. The reagents have expiration dates of 05/2019 and 06/2019.
The other test kit is the Salifert nitrate test kit. According to its documentation, it also reads as ppm of nitrate (NO3-), and is the one claiming 3ppm. It expires 09/2016.
Sorry, but the API test *I* own, is clearly inaccurate. The idea that the municipal supply for Baltimore (which provides a large part of central Maryland) is 20ppm (twice the EPA action level) and has been undetected is absolutely absurd. The municipal testing suggests Nitrate levels in the 2-3ppm range, which is on-par with the Salifert.
This is why you should calibrate your test kits. I admittedly have not calibrated mine, but only one of these test kits is reporting numbers that are at all believable. I do not trust or use the API test anymore.
API brand new kit, got it last week. 5.0 to 20. The discus tank and standing water seemed to be at 40 today. Everyday it's slightly different result for the well water.
Any other plant that soaks up nitrates well?
1) do yourself a favor if you can and make a set of test solutions and check your test kit.
2) pretty much anything fast growing is a good nitrate consumer. I've seen some recommendations around here of using floated hornwort for this purpose.