Mysterious Cardinal Tetra death!!!??

I just got 6 card. tetras on tuesday and they were realitively fine. That night i found one dead!! Cause of Death? I dont know!!! so wedensday morning i found another one dead!!! later that day, i nothiced one's tail fin was completely gone and i struggles to even float and then it died!!! so you can imagine wat happens next and pretty soon im down to two cards. in 2 days. i cannot figure out why they died. and also two of my guppies that i have had for 3 weeks died. My tank is 5 weeks old, gone thru its cycle and everything!!!! amonnia is 0 ppm, nitrite is at 0, and my pH is at 7.4. Wat is wrong!!!!