What is needed on a Digital Controller

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Sort of following through on another thread, what is really the minimum required on a Digital Controller? What got me thinking is hopefully soon I'll been setting up a Ca reactor so thinking might be a good idea to control the unit by monitoring the pH of the effluent.

I have a 145g that is totally functional with two timers, one for the NO fluorescents and a second for the MHs and a hood fan. The basement sump return pump and skimmer are on the same switched power-bar and the CL is plugged into a separate receptacle. Heater runs on it's own internal thermostat.