I've never kept Cardinals, so take my opinion with a grain of salt:
I know Cardinals have a reputation for being difficult to acclimate. I noticed fish stores lose most of their Cardinals within a week of arrival, because they don't acclimate them properly. So if you've bought from a fresh shipment of wild-caughts, you risk losing most of them. I'd also be concerned about your huge periodic water changes in a small tank, especially if it isn't understocked. If Cardinals are very sensitive to any swings in water chemistry, could that be what's hurting them?
Otherwise, since worms were already suggested, check for Neon Tetra Disease. Does the musculature near the base of tail, or especially near the dorsal fin, look milky before the fish dies? (I know it's hard to tell on a colored fish. Try shining a bright light behind a fish - healthy muscles should look clear.)