They are very delicate fish. They require a tank that has been up and running at least
3 months, preferably 6, with very low nitrates (and of course no ammonia or nitrites). Since they are caught in the wild, they need very soft, acid water to match their natural habitat. If they are tightly shoaling after a couple day "break-in" period they are too stressed and won't last long. I found that a solid black background, java fern, plenty of hiding spaces, and tea-water brown with tannins is ideal for the little buggers (with the added benefit that these surroundings really bring out the color. They need to be bought from reputable dealers who understand them. I got all mine from a lfs
called Rick's Pets, and they lived for a good time. I had a steady population for about 8 months, and then put hatchets in which introduced ich and nuked the tank.
Another option is to look for tank raised cardinals. They are usually advertised as such, and cost more, but are hardier.
As far as discus and cardinals, Rick's Pets has a beautiful discus tank set up with a school of cardinals, and they are fine.
Google "Amazon black water" and "Rio Negro" and you'll find a wealth of information. I have a picture of my tank setup in my gallery (it's the dark one with the skull).