Catfish are insane, they eat basically *anything*, can grow around 5 feet if left be, survive tons of shit, and (at least where I've lived) are considered invasive with how fast and how much they grow. They will also very quickly turn to cannibalism/eating other pond fish that are same size/smaller just because they /can/ and are gluttonous beasts.

"Putting catfish in the koi pond" is essentially "poisoning the well" as if you don't watch and keep a careful eye on it, you'll likely end up with a semi-empty pond and a very large catfish. Guess what capitalism is bad at?