It is unusual to see different substrates in different areas of the same fish tank. Some reports say some fish might act less timidly with darker substrate. It can be read as some fish feel safer with darker substrate. It is possible the brighter substrate in your aquarium is making the Cardinals feel unsafe. As a result, they hide in a corner over the darker substrate when you have given them a choice. You can mix up the two substrates to have the entire fish tank floor look the same. Increase the number of Cardinal Tetra might help.
You might want to wait a few more days first. There is nothing to worry about fish not eating for a few days. They can live for weeks without food. Give them some time to get used to their new home. If the problem does not go away by a week mark, then you can try some of the suggestions.
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20 gal tank - good mix of community fish ?
