When I first tried cardinals, I had them drop like flies. I got them right off the truck, special order at the lfs. I talked to an old-timer hobbyist who told me to feed them baby brine shrimp, preferably live, but frozen would do.
I tried them again, same lfs, feeding frozen bbs starting the second day, and had no losses.
That was just my experience.
Sent from my nerd device.
I've been feeding the discus frozen brine shrimp and sometimes the Cardinals grab one. Same with the blood worms that I give them. This was my first try so maybe next time I'll do a quar. tank and try harder not to kill them.