You've got two issues -- as others have pointed out, all three members of the neon group do better at slightly higher temperatures, though they'll survive well, even in their native range, in the lower 70s. The bigger issue I see is the "soft ish." If you're in this area, your water has a TDS of about 100 mg/L -- depending on where exactly you are in the area. (Potomac water sources are higher, Patuxent is lower, and some jerks in Southern Maryland have water that's ridiculously soft). If you're dealing with Florida bred cardinals or, more likely, neons, that's fine, they're adaptable, but a lot of these fish are wild caught, and they simply cannot survive in our relatively hard water for long periods.
I would find out your source for the fish, and go with Florida bred ones. Asian bred ones are similar to wild ones -- they're grown in softer water, and they adapt poorly.