You will read many erroneous things about how fish can adapt, even though many are quite adaptable to many varied conditions, some are not, especially those wild caught in black water.
Some are just not adaptable, after billions of years life in a specific environment (like Amazon black water rivers), which may include seasonal changes , like a period of time where tannins permeate the flooded forest, and unless they have been bred for years under aquarium conditions, and unless you can adapt your tanks to at least some of those conditions you may be fighting a loosing battle.
Some adapt when young, but get long term chronic, scarring maladies as they age.
Captive bred, in your area's tap water, would be the most reasonable way to go.
There were aquarist friends in my area who were successful with, and able to even breed cardinals.
These people mixed RO with the hard tap water, and mixed tannins in when doing water changes.
They collected leaves in fall, soaking them for long periods in buckets to extract the anti-bacterial tea like water. I did the same for certain species of killifish.
