I got the AC3 because it can monitor and control two pH devices, which allows me to check the tank's pH and control the Ca reactor's pH. Well, yesterday night, the pH values were wildly off (around 2 and 10+). I knew something was up. Turns out, there is a very minuscule electrical leak/grounding issue in the water interfering with the readings. After systematically unplugging each submersed device, I found it was a pump gone bad on my Ca reactor. Without this monitor, I would've never noticed until it developed into a more serious problem. 8)