Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Neons have been raised in captivity for so long that they are OK with much harder water than their ancestors.
Similarly, Guppies have also been raised in captivity for a long time. The fanciest are the most delicate and still demand the hard water of their ancestors. The 'mutt' Guppies available almost everywhere are more tolerant of a wide range of conditions.
Still, the biggest Guppy I raised was in my brackish water tank, so I am inclined to try giving them harder water, but salt is optional.
However, if the fish are doing OK as is, then perhaps it is best not to change conditions. Maintain the tank the best you can, improve whatever is possible and go with plenty of water changes.