good morning slider..
I read your aquarium info, and see the tank has been running for six weeks. I think the reason you are getting ammonia readings is due to the tank not cycled yet. Your platies for some reason are sensitive to the ammonia and probably what is killing them off.
Yes, the cardinal tetras are very sensitive and should not be brought into a immature tank, but so far so good... and that is great.

Since you have the fish in the tank while it is cycling I would keep up your water changes to keep the ammonia down...
good luck with your tank!

~ kate