Thank you for the replies!
I intend to get a new API KH GH test kit, since mine are expired, but this gives me a method to establish a baseline to see how accurate my readings are.
The UV sterilizer seems like a prudent purchase if you’re going to keep these kind of fish.
An airstone was also recommended, which I will add to the list.
No DOAs that I can see in the tank this morning. Cross fingers!
If things stabilize I might get some rummynose tetras, which in the past have been very active in my lit tank, to see if they can make the Cardinals more comfortable In coming out. An aquatic Judas Goat, so to speak.
Again, I hope I can keep those I have left alive and really appreciate the input.