Why can someone say that one fish is more deserving of being used as a feeder, than another fish? Excluding the feeder being endangered, diseased, dangerous or otherwise.
All fish are food for something. All fish of size are food for humans. A predatory fish isn't likely going to look at a three different species of fish and only it one that it specifically chooses. It's likely going to eat em all in order of easiest catch.
Aside from the catfish being oversized, I don't understand how people can say that a goldfish should have been used instead. If I moved to South East Asia I'd eat cats and dogs, but here they are pets.
Nutrition should really be the biggest concern, but most aquatic predators likely don't have too much trouble being provided with the required dietary needs.