I have had a much tougher time keeping cardinal tetras alive than neons or glowlights. I had 12 cardinals, 5 neons and 6 glowlights in my tank before I decided to change my gravel. After the change I lost all the cardinals but lost no neons or glowlights. I thought the loss was due to my nitrites skyrocketing after the gravel change. Interestingly, they didn't die all at once, usually 1 per day. I did 50% water changes once/week until the tank cycled again, which took about 5 weeks.
Curiously, the dying cardinals all followed the same pattern. First, the whole cardinal shoal would look perfectly fine. Then, maybe just an hour later, I would see one cardinal off by itself, away from the school, but still looking fine. Then, maybe an hour or two later the loner would be up near the surface, swimming weakly. Then, soon after - kaput.
So, w/ my tank chemistry right again I bought 2 new cardinals. The next day was water change day and w/in 6 hours both cardinals were dead. Same behavioral pattern as above. I tested the water and ammonia = 0, nitrites = 0, nitrates = 10, pH =7.0. My discus are happy, so my water must be at least alright.
Also, being new at this, prior to the gravel change (and buying discus) I seldom did water changes and never lost a cardinal.
Cardinal tetras are one of my favorites and I look forward to having a nice school of 10-20 again. Any advice on this problem would be appreciated!