What warnings do the goldfish people give?
Imagine the heat.
Yes, the heat. And the cold. And what if they breed? And what about aquarium salt, hmmm? (I don't use aquarium salt for any of my fish.)
Ponds are for koi. A galvanized tank will rust. You have to have a power filter - a bog filter isn't sufficient. What about predators? What will you feed?
Plant fertilizers for outdoor plants will be bad for the fish. (They don't get the notion of the self-sustaining eco-system...fish poo equals good plants, good plants equal nicely filtered water.)
And on, and on, and on.