A 12 long AND a boston terrier? I think we're tank soulmates. Except I have, ahem, two of each.
As far as softening the water you can mix tap and RO, use all RO and remineralize it, or you can do peat. Here's a thread where they made a peat filter: My DIY peat filter
You could also just use a substrate that buffers the water. ADA aquasoil, Azoo plant grower, and Up Aqua soil are a few options.
ETA: You could also just use your tap as is. I don't know about the cardinals, but I have cories, otos, and other soft water fish that are fine and healthy in my tap which sounds very similar to yours. It reads 7.4 or 7.6 depending on which API test I use and has a gh of 12.