All fish in tank stressed and dying.

[jeaninel]

Sorry to hear about your losses. I have found both Cardinals and rummies to be very sensitive fish in general. I always lost a few when I would buy a group. I second the QT tank suggestion. Having to treat a 225 gallon tank would not be fun. I always QT any new fish for at least 4 weeks.

Oh, wanted to add that I've found the rummies that did make it past QT were very hardy in the long run. My experience with Cardinals has not been the same. Never been able to keep them for more than a few months.

 

[duanes]

One thing you may want to consider, most cardinal tetras are still wild caught, and come from blackwater rivers (like the Rio Negro) that have a pH averaging around 4. (your 6.6 is about 260 degrees less acidic). If wild caught, and conditions are slightly out of the norm, deaths may be eminent.
Getting fish more suited to your water, would be an easier road.
Beyond just pH, the tannins that make black water rivers black, have antibacterial qualities, so if your tank does not have those tannins, normally benign bacteria can become problematic for black water fish.
You can produce your own tannins by soaking leaves, or peat moss, and adding the tea colored water during your water changes.
You can also put peat in mess bags, in filter, or hang tea bags in the outflow of filters to get the antibacterial qualities.
I used to soak fallen leaf littler from "untreated" trees in my yard, and even add leaf littler to tanks for these black water fish.