Give the KHD a couple days and tests to settle-in.
What you're looking for is back-to-back testing consistency within the acceptable tolerance of +/- 0.2.
If results are wildly fluctuating (8.0, 4, 10, etc.), then you know the probe should be calibrated. A quick way to test would be to put the probe in the 4 and 7 calibration fluids and see what results you get out of it. If it shows within the acceptable tolerance, probe calibration is not necessary. In that case, check for air bubbles or a contaminated water sample.
Another important thing worth noting...conventional test kits and the KHD do not use the same testing protocol. With kits like Hanna, you have more factors to consider than what you'd get from the KHD. To name a few, the test result from a Hanna checker can be skewed by fingerprints, dust particles around vial or inside the sample, actual size of the water sample added, age of reagent powder, how well vial was mixed...etc. A KHD does not have to consider these kind of variables because it does not follow the same testing procedures.
KH levels can swing for several reasons, not just coral. Even on a bacterial level, KH results can be changed day-to-day.
If need be, a bottle of reference fluid will help fine-tune the KHD with it's adjustment feature.
