I've never seen cardinal tetra disease; from the photo of the whitish area up near the spine, this looks much like neon disease which is a parasite that makes tiny cysts in the fishes' bodies. Unless there's a brand-new treatment, once they have it, they're more or less doomed. In 50+ years of keeping fish, the one thing I know for sure is that it is ESSENTIAL to remove sick fish from the tank. If a sick fish dies, the others will immediately nibble it & will get sick. Past experience with tetras including cardinals: soft, slightly acid, clean water w/ no ammonia, nitrite or nitrate (I've always added peat to the filters on my Amazon tanks). Low stress & lots of plants. Remove & humanely euthanize sick fish... that's what I've done w/ good success; my fish almost always expire from old age unless I accidentally introduce a pathogen to the tank, which can happen to the best of us. Best of luck to you.