🚀 go-pugleaf

>Does anyone know of electronic/electric devices in the market that
>are made/could be modified to measure water hardness and pH?

Such devices are very common in analytic chemistry done in the field.  Electrodes
are available whos potential is dependent only on the concentration of a single
type of ion.  This is true within limits.  There will always be some interference.
Potentiometric pH meters are pretty common lab devices.  Perkin-Elmer and
Beckman Instruments make them.  I think Edmund Scientific also  sells them.
Measuring the hardness of water means determining the magnesium and calcium
ion concentrations.  Ion selective electrodes for these exist also.  Try
posting to sci.chem.  You will probably get more response.  I am not in
analytic chemistry(I am in computation chemistry).

                                                 Brent H. Besler