Did you aquire the cardinals recently? I have also found initial acclimation is difficult(they are wild caught and have gone through a lot of stress) and had a lot of them die. However, the survivors will live forever if water quality is good. So it might not be your fault and don't give up hope yet. Next time I get more cardinals, I will put them in quarantine/hospital first, much easier to treat that way.
Flourite, wet/dry, AHSupply 302w PC, PinPoint controller
15w T6+15w T8
40w T6 ODNO4x, no CO2. Discus grow out.
10gallon Tonina tank, 2x15w T6 ODNO 4x
I will do aquascapes after I collect all the plant species and grow them to perfection.