If you follow the setup instructions that come in the box there is a step where you have to calibrate the probes. There are packets of standard solutions in the box as well. The Apex controller is an amazing tool, when properly calibrated, and programed, If your an Apex noobie, just use it to monitor, don't jump in and immediately use it con control EVERYTHING. Start with your lighting. Then as you get comfortable add more controls. When I started using my Apex my tank was still cycling, and was an invaluable tool in me knowing for sure the tank was ready for lifeforms. At that time I only had lighting and heater controls setup. ORP, pH were just monitoring, and still are. This weekend I will be setting up Ozone controls via ORP monitoring.