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.
Well as far as cold goes , plenty of people in NY keep Goldfish in ponds which have the top freeze for months.
The Miami fish farm here have Koi in a small pvc type pond as you enter and they sell plenty of goldfish in outside vats.
You could put some large pots or plant shrubs,etc directly around the tank so that the metal doesn't absorb sunrays, heat.
I would think that you will still need to do a water change now and than even with a bog filter ?
I imagine you have seen this site before and would think temps similiar to yours.