Thought I would share I how monitor and control the alk in my 20,000 gallon reef . I use and off the shelf alk controller that I built into a custom waterproof box . I take the signal from the alk controller and covert it to a signal that my plc can use then the plc controls the co2 flow to the calcium reactor via a pwm signal . in stared of an on and off signal it ramps up and down so I do not get the saw tooth effect seen inn most tanks .