Cheap and works.
People debate the accuracy of this all the time when compared to the higher end test kits. for me, it simply works. the resolution of the testing is to 10 ppm and other products go .5 ppm. i'm personally okay with knowing i'm either 420 ppm or 410 ppm weekly. the point is to graph and understand the trend line over time. if you're trying to track specific daily fluctuation you're better off going with a higher end kid. for me it works and is simple to use for my application. tip: 10 drops of bottle #1 and shake. when you start bottle #2 you'll start seeing a trend based on your tank consumption. for my tank, i'm consistently at 400+ ppm. so for bottle #2, i put in 19 drops at the same time and shake. then add additional drops until i see the color change. don't add a drop and shake starting at the beginning or you'll be there all day.