Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you want to keep fish that demand soft water, especially to breed these fish, then you will want RO. The water softener is adding sodium to the water (usually) and the RO will take that out.
If you are keeping average sort of community fish that have been bred for many generations in captivity then the 50/50 blend looks pretty good.
If you are keeping hard water fish (many live bearers, certain Rainbows and their relatives, Rift Lake cichlids, some others) then pure well water, or well water with just a little softened water would work just fine.
Might be worth getting a TDS meter. That will show you how much other things are in the water besides what the GH and KH tests are showing.