Water testing question

I use the API water testing kit which I bought a few years ago. According to it, nitrate seems to be always present no matter how many times I change the water. It's a 55G tank with about 75lbs of live rock and 50lbs of live sand and the water always looks crystal clear with all the residents doing fantastic. I recently got my water tested at a local store and I was told that nitrate is at 0. Now I'm confused.

 

maybe it's old!?

 

Most likely expired.

This thread explains how to find out

https://www.3reef.com/forums/water-chemistry/those-who-use-api-test-kits-fyi-48335.html

 

Thanks!