Hi..

I've kept neons and cardinals for years in hard water.
Lots of people have.

Are "cardinals" == Tanichthys albonubes..?

They are still hard water fish(es).

The thing about acid water is it keeps to a certain
extent, bacteria at bay. I always suspect clenliness, not
pH or hardness when people have trouble with blackwater
fish.

Soft blackwater is clean because bacteria do not prefer such
conditions. And this is why soft and clean water is ideal
for several soft water species..
--
cu
Marco