Hey there, I specialize in Tetras and other small schooling species.
First of all I'd like to admonish you for not qurantining them for at least a couple days to a week before introducing them to your main system. What, you want all your fish to catch Ich or other microbial diseases?
Now I have over 40 Cardinals in my 200g schooling/planted tank and I've never experienced mass deaths like that, no matter what I did to them/introduced into the water.
You should really post water parameters if you want specific help... But from what I can gauge, unexpected deaths when introducing new stock usually comes from stress. Did you acclimate them properly? Are they being harassed by other residents? How were they acting before you got them from the store?
Any information is helpful in determining what went wrong.
Last edited by AVN; 10-06-2012 at 08:22 PM..
Reason: Additonal Information Added