Keep your pH probe calibrated!
Neptune Systems single-use pH calibration packets make calibrating your pH probe simple and fast. Just tear open the top, and a stick your probe into the solution when performing the calibration procedure.
Over the course of a couple months, it is common for pH probes to not read exactly correct and that can be fixed by calibrating your probe. Almost all monitoring devices will allow you to calibrate your pH probe, and a majority of them will let you calibrate your probe to a high and a low point. For saltwater aquariums, it is best to use a calibration solution with a pH of 7.0 for the low point and 10.0 pH for the high.
1x 7.00 pH Single Use Calibration Packet