I would calibrate your API test kit. I did that with mine and my colors were way different than what the color indication chart said it should be. You can make standards using distilled water and KNO3 I believe. You had a certain X amount of KNO3 to X amount of water to get a certain ppm. Then you keep cutting the mixture down by 1/2 to get each amount that is on the color chart. After doing this, I got a 20ppm color for a 5ppm standard, 10 for 40, 25 for 80, and 50 for 160. Hoppy wrote up a good thread on this, if you search for, "Calibrating Test Kits - for non-Chemists" within the TPT forum, you will find it. Hope this helps!