API seems to work well with higher nitrate levels, but Salifert has a much lower resolution. The first color change on an API card is 5.0
ppm. With the Salifert test, you can read down to 0.2ppm.
You might want to test that tap water with a nitrate test kit that has a lower resolution, like the Salifert kit. I'm thinking part of your problem is coming from your source water, but it's just flying below the resolution of all your test kits.