Does it control micrisiemens?
I would guess it was a fresh water controller. I would offer that one could argue it is a conductivity controller because an indirect result of fresh water control, a possible corresponding result could be a chage of conductivity.
I would call it a conductivity controller. I think that the OP wants to monitor conductivity, not use a closed loop control for it.
You are correct. Calling it a micro siemes controller would be like calling a temperature controller a degree C controller.
If you are controlling the temperature of a jacketed vessel by controlling the cooling water flowing through the jacket, it would be a temperature controller, not a flow controller. A flow controller implies that the flow is measured and controlled. Technically, you could have a temperature controller cascade a remote setpoint to a flow controller and have both a temperature controller and a flow controller.
Back to the OP topic. You are looking at a cost of $100 to $150 dollars just for the sensors. ~$70 for a conductivity probe, ~$30 for a pH probe, and ~$10 for a temperature probe (assuming that the temperature cannot be read from one of the other sensors).
It may be more cost effective to but a unit instead of building one. You tend to get a lot better prices when you buy in the thousands instead of one by one.
If you are set to build it yourself, start researching the available sensors that will work with the arduino either through a shield or direct connection. Then look at some similar projects on Google to get you to a good starting point.