10-03-2012, 11:10 AM
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Vallisneria thrives in the hard water of the rift lakes in Africa, and they can use bicarbonates as a source of carbon if CO2 is minimal, and some sources suggest they prefer bicarbonates. So I don't think there is much doubt but that they do best in harder water rather than soft.
But, as always, there are other factors at work. Light has to be good, and other nutrients must be present. In soft water, carbon would perhaps be an issue.
And, as i remarked previously, the plants in the photios look pretty good, and only the owner will know the extent of their decline.