yes, i agree, by their nature they will soften water, ...
but to what degree ?, just general plant metabolism ?, or are there plants that could have a measurable effect on the water ?
i have heard some plants (and plant roots in general) produce chelating compounds that can attach to nutrients and make them more available for uptake, ... but roots are in the substrate and not the water column
or there's duckweed and other floating plants, ... but would they need to produce any chelating compounds as everything is already floating ?
did a search yesterday on water softening, that's almost an impossible search (like the search so many months ago on diseases & infections rotifers can get, ... all search results are human related), ... with water softening all search results are using mechanical devices we buy ...
we are one of ... a billion creatures on this planet, we're not that important. ... and it makes searches so difficult at times.