a pH monitor is obviously very handy with a calcium reactor. I would recommend a good one even without, they are more accurate than color change test kits and will give you a better idea of the pH rise/fall(normal) over the course of a day.
I believe most of the better aqua controller devices have ports for pH,orp, temp etc monitors and have the ability to control devices that can maintain params within a certain range, ie activating a Kalk reactor if pH drops below a setpoint, helping to bring it back up. The normal trend in a closed system is pH decline do to CO2 both from air /water interface and the respiration cycles of fish and inverts.
ORP basically indicates levels of dissolved organics in water column and is usually modified by activating an ozonizing device. These don't seem as common as they once were, but it seems that most hi end controllers have provision for dealing with this as well as programmable timers for powerheads/pumps, lighting , and temp probe inlets that can be used to drive fans or cycle a chiller off/on
All sorts of neat things to make life easier. Some have the ability to hook up to an old PC, like an old 486 mghz and with available soft ware can automate a lot of tasks that lead to stable tank. Heck some can even page you at work if params get outta wack.
But back to the basic issue, a pH monitor along with a hi quality calcium and alk test kit will make it easier to monitor those areas, and they are all related so the better the data the better the result.
As for salinity, I am not sure how accurate they are, and I dont know of anything that you can hook up to it that would make changes to maintain a set SG. I would invest in a good refractometer and monitor as needed, HTH
