The API GH test kit isn't very good, in my experience. It might work ok right after you buy it, but it might not. And, not too long after that it likely won't work at all. I switched to a Red Sea test kit, which worked a lot better. But, I only used that one a couple of times, then just left it on the shelf, so it may not work well now either. When a GH kit works well, the color change is very easy to see, and you get an easy to interpret reading. When it doesn't work well, you may not see any color change, or it may be so light in color you can't be sure if it changed or not.