General hardness is the divalent metal cation content of the water. In most water this is comprised primarily of calcium and magnesium. Soft water has a low concentration of dissolved divalent cations, hard water a high concentration.
The native environment of some plants (Cryptocoryne, Aponogeton, etc.) is soft, whereas others (Sagittaria subulata, Riccia fluitans, etc.) are more acclimated to hard water. In order to replicate these environments it may be necessary to adjust hardness.