Hi all,
Roy it doesn't directly measure hardness, but because you start with RO you start with only 1-2 ppm TDS (theoretically it should be 0 ppm, but it rarely is) you can add the buffering salts to give you a datum TDS value.
If you start with the mix at <
James' Planted Tank - Re-mineralising RO Water> for 25 litres, and just add that to 25 litres of RO you can measure the TDS and at that value you know have 1dKH and 5dGH.
Other salts you add like KNO3 will raise the TDS, but not the dGH (just multivalent cations) or dKH (just carbonate anions). All TDS meters measure conductivity, rather than TDS, and just multiply the conductivity in microS by ~0.55 to approximate to ppm TDS.
I use an even less precise method for soft water fish, I cut my rain-water with tap-water (about 17dKH) to give about 70ppm TDS (100 - 120 microS). I don't bother measuring anything else, and I use the Duckweed Index to feed the plants when they really need it.
This method still leaves the water too hard to breed "Blackwater" fish like
Dicrossus filamentosus, but it is fine for their maintenance.
cheers Darrel