[reefgeezer]

The Hanna tester is pretty accurate...Less than +/- 0.1 when reading 8-9 dKh. That rating is just for the hardware. It doesn't take into account measurement variability created by the testing process. It's not that you are doing anything wrong. It just means there are variables in every testing process that impact the accuracy of individual tests. Things like cleanliness and clarity of the vial, cloudiness of the water, slight variance in water volume in the vial, small variances in the reagent, and etc. can change the reading a few tenths.

You have to get away from comparing individual tests. Start a spreadsheet and record each test. Then analyze the trend rather than the delta between individual tests. Good decisions are usually based on the longer trends.