I concur, Cardinal Tetras need a minimum tank size of 23in x 12in (60cm x 30cm), which is a 15g tank. (Assuming it's a common 15g tank: WHD; 24in x 12in x 12in)
Paracheirodon axelrodi – Cardinal Tetra (Cheirodon axelrodi, Hyphessobrycon cardinalis) — Seriously Fish
What are your water parameters? (ammonia, nitrite, nitrate, pH)
Another thing that caught my eye was that you said you had Mollys, but then got Tetras. How hard is your water? (Tetras are soft water fish, while Mollys are hard water fish. If we keep fish in to hard or to soft water, it can damage their immune system and cause reproduction issues.)*
*Which may be the cause of your fish dying, but it's not likely...