Captive bred are probably your best bet, but if it were me, with your tap water (just like the water I had) I wouldn't go with any Amazonian species at all (this may just be my innate laziness), but unless you are prepared to augment your water with tannins, or go some other approach, like every other day water changes, mixing with RO or rain water to reduce the liquid rock by half, maybe even adding old tea bags to HOB filters. A more reasonable approach might be to go with the flow instead of fighting it, and get fish from the Great Lakes of Africa, or Central America would be a better fit for your conditions.
There are tetras from Central America(Astyanax, pH 7-9), tetras from west of the Andes tolerant of pH 7+ and Africa (Congos, pH 6 ish, yet again captive bred pH 7ish )much better adapted to your tap
Just a thought too, I believe your tank has been compromised with whatever killed the cardinals in the first place, and may need to be disinfected, and restarted before adding new fish. Many pathogens can go into an inert dormant phase for a long period of time, even without hosts.
There are tetras from Central America(Astyanax, pH 7-9), tetras from west of the Andes tolerant of pH 7+ and Africa (Congos, pH 6 ish, yet again captive bred pH 7ish )much better adapted to your tap
Just a thought too, I believe your tank has been compromised with whatever killed the cardinals in the first place, and may need to be disinfected, and restarted before adding new fish. Many pathogens can go into an inert dormant phase for a long period of time, even without hosts.
