Knowing what your aquariums pH level is a vital component to understanding what is going on in your tank. Testing for pH is straightforward and only takes a few seconds with the Hanna pH Checker Plus. With any measurement device over time, the readings may start to sway, and calibration may be required to get accurate readings again. Most instruments have a single calibration point in the center of the scale you will be reading, but the Hanna pH Checker lets you use two set points allowing it to be precisely calibrated to a particular range. Calibrating the pH Checker Plus only takes a minute and you can choose if you want to calibrate between 4.0 and 7.0, or 7.0 and 10.0. We highly suggest calibrating any pH probe to the range that it will be reading. For most reef tanks, you will want to use 7.0 and 10.0 set points, and for calcium reactors and most freshwater aquariums, the lower threshold would be ideal. A couple of tips when calibrating the Hanna pH Checker Plus is always to use new calibration solutions, and to rinse the probe end in RODI or distilled water between sachets. We like to calibrate our probes once every one to three months depending on how often we are using the particular probe.