I am sure you are doing it but with the API test kit it is very important to follow the directions for the nitrate test. There are crystals in the 2nd bottle of solution and it has to be shaken very good. I shake mine for about 30 seconds vigorously hitting it on my leg before I add the first bottle. Then I add the first bottle and shake it again hard for about another minute. If the crystals are really knocked around in the bottle you will not get an accurate test.