I use API as well, but I'll be the first to point out that they're not the best in the world in terms of accuracy. At bare minimum, I'd go out and get a new API test kit if I were you....and if you don't trust API, try a new brand test kit.
Also remember that how you care for your glassware is very important -- it should be cleaned thoroughly between tests. In a perfect world, one would wash their glassware with glassware detergent and water, rinse with DI water, and dry with acetone (displaces the water and evaporates quickly).....but I know of nobody who does this. At the very least, rinse it well with tap water, followed by a couple rinses with RODI water if you have it on hand, right after use. Before the next test, rinse once with RODI water and again in the tank water. Make sure you're shaking your ingredients and following the time requirements correctly, as the color DOES continue to darken the longer you wait. Also avoid touching the test tube with the applicator tip.
All these detailed directions just for testing the darn water. Isn't this hobby FUN!