20 gal tank - good mix of community fish ?

Sorry, I did not read your post correctly. I thought you were going to add another 5 Cardinal Tetra the next Sunday.

Something is stressing them. A few things you need to check into.
For example, is the water current too strong? Is the heat from the heater evenly distributed? What is your water temperature anyway?
I'd also recommend to add a background to cover up the back of your fish tank. Add more hiding places can also be helpful. Fish are more likely to act normal when they feel safe.

By the way, haven't they eaten anything at all since you got them? Always stay in the corner?

Please show me a photo or two of the situation.