Wild caught cardinals require time and care to acclimatise to a new tank setting. In the farms they are kept in acidic water.
Most often, the problem is not being to feed them, or entice them to eat dry food like pellets. Wild-caught cardinals will first require live foods and slowly switch to a mix of dry and frozen foods if necessary.
My advice is to stay away from XXL Cardinals, if you cannot match their requirements. The small cardinals are tank-bred fish from farms elsewhere, so they are more or less used to other conditions.
The fin rot is almost always due to a bacterial infection caused by an injury or damage to the fins. They might have had the problem back at the dealer's place, and when you moved them into the new tank, it caused them too much stress and the bacteria managed to overpower their immune system.
This is why most people who buy wild fish, will always quarantine their fish before putting them into the display tank. It is not easy since wild fish can come with all sorts of parasites/diseases in and outside.
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