Why do you think it was the dragon stone?
Most cardinal tetras are wild caught, and may carry parasites.
Since you have not stated what your water parameters are, but...., if your tap water is hard, this can cause enough stress in cardinals, that a normally benign bacteria can become a problem (especially if there are no tannins in the water), and in the confines of a tank, become epidemic enough to also infect the cichlid (especially one) that also prefers soft, tannin rich, low pH water.
Water are your tap water parameters? pH, hardness, alkalinity
which can come by calling your water provider.
Without the tap parameters, and your tanks water parameters,
Ammonia, nitrite, nitrate, pH.
it will pretty hard to reasonably diagnose the problem.