With any of the test kits we can get at a reasonable price the accuracy is questionable. The tests for NO3, PO4, Fe are notorious for not being accurate at all. If you want to use them, and want to rely upon them, you have to calibrate them and make sure they will read accurately in the range you are interested in. It does no good if the lowest usable reading on a NO3 test is 20 ppm, for example. Since the ppm of iron we want is so low, it would be asking an awful lot for a cheap test kit to be able to read that low. And, the same is true for PO4.