Milwaukee Calibration Solution sold in boxes of 25.
Almost all pH monitors and controllers will require two points of reference to accurately monitor the pH that the probe is reading. You will want to use calibration solutions that are on either spectrum of the pH you intend to monitor. For reef tanks where the pH is normally 7.8-8.5 then you would want to use calibration solution 7.0 and 10.01. For calcium reactors on saltwater tanks where the pH is normally below 7.0, you would want to use 4.01 and 7.01 for calibration of the probe.
4.01 pH - The lower threshold reading your pH monitor or controller will require for use with calcium reactors for saltwater aquariums, solutions below 7.00 pH and also most freshwater aquariums.
7.01 pH - Neutral pH calibration packet. This is used by almost all pH monitors and controllers for calibration.
10.01 pH - The higher threshold reading your pH monitor or controller will require for use with saltwater aquariums or solutions above 7.00 pH
Calibration packets can be stored in a cool dry area for up to 2 years.