... Now to my question.
Question: I tested for nitrates with an API fw test kit. ...
I'll prefix this post by stating up front that some of this is my opinion, and it's likely others will disagree.
I was going to test my water for nitrates, and found that the "famous brand", but not API, test kit I was using had gone bad. I stopped by my LSF the next day, and found that the only nitrate test kit they had in stock was API.
Now I admit that I'm not a big fan of API test kits, but I figured it would at least give me some idea of my nitrate level. I did the test and found that I got a reading of 0. Since I had just added dry ferts for nitrate the night before, I knew I had to have at least 10 ppm of nitrate. Obviously something was very wrong with the new test kit, and I was "rather disappointed". Yes, I know that for an aquarium grade test kit to be accurate, you do need to calibrate it, but I should have gotten some sort of reading.
I then figured that I might as well spend the money on a good test kit, I ordered a LaMotte Nitrate (model 3354) test kit. I tested the water, and got expected results. Now to be fair, the LaMotte test kit costs about 10 times what your pay for an API test kit.
I have come to the conclusion that most of the aquarium grade test kits, are at best fair, and at worst garbage. From here on out I'm getting quality test kits for anything that I want to monitor closely.