The first step when testing for nitrates and phosphates is to calibrate the test kit. Mix some DI or distilled water with enough KNO3 to get a known 100 ppm, for example of nitrate. Check it with the kit. Then mix some of that water in equal parts with DI or distilled water, to get 50 ppm and check that with the kit. Repeat until you are below the range of interest. Now, you will know what your test kit readings are telling you. The kit may be dead on right from the box, but it may also not be.