Adding chain LFS fish directly to an established tank with expensive fish is an astoundingly poor choice.
LFS's generally share water between a large number of tanks, so any a single diseased fish could easily contaminate many many others. Big chain LFS's tend to have more unhealthy fish, on average, than a mom n pop LFS.
My death rate over a couple weeks, when buying from one of the big chain LFS's, is between 1/3 and 1/2 of purchased fish. The first time I bought fish from one of them, added them directly to an established tank, and lost not just all of them, but 80% of the previous tank occupants, was when I purchased a 10g tank for quarantine purposes.
Quarantine all the cardinals ASAP. Not sure how much it will help your current troubles, but keeping a QT tank for all new purchases will save you many headaches (and money) down the line.